Add new SKIP macro for skipping tests at runtime (#2360)

* Add new SKIP macro for skipping tests at runtime

This adds a new `SKIP` macro for dynamically skipping tests at runtime.
The "skipped" status of a test case is treated as a first-class citizen,
like "succeeded" or "failed", and is reported with a new color on the
console.

* Don't show "skipped assertions" in console/compact reporters

Also extend skip tests to cover a few more use cases.

* Return exit code 4 if all test cases are skipped

* Use LightGrey for the skip colour

This isn't great, but is better than the deep blue that was borderline
invisible on dark backgrounds. The fix is to redo the colouring
a bit, including introducing light-blue that is actually visible.

* Add support for explicit skips in all reporters

* --allow-running-no-tests also allows all tests to be skipped

* Add docs for SKIP macro, deprecate IEventListener::skipTest

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

4 files changed, 86 insertions, 4 deletions

diff --git a/docs/Readme.md b/docs/Readme.md
index 52eb64a5..126ee1be 100644
--- a/docs/Readme.md
+++ b/docs/Readme.md
@@ -11,6 +11,7 @@ Once you're up and running consider the following reference material.
 * [Logging macros](logging.md#top)
 * [Test cases and sections](test-cases-and-sections.md#top)
 * [Test fixtures](test-fixtures.md#top)
+* [Skipping tests at runtime](skipping.md#top)
 * [Reporters (output customization)](reporters.md#top)
 * [Event Listeners](event-listeners.md#top)
 * [Data Generators (value parameterized tests)](generators.md#top)
diff --git a/docs/command-line.md b/docs/command-line.md
index 6fc99f59..e7a448b1 100644
--- a/docs/command-line.md
+++ b/docs/command-line.md
@@ -561,10 +561,10 @@ processes, as is done with the [Bazel test sharding](https://docs.bazel.build/ve
 
 > Introduced in Catch2 3.0.1.
 
-By default, Catch2 test binaries return non-0 exit code if no tests were
-run, e.g. if the binary was compiled with no tests, or the provided test
-spec matched no tests. This flag overrides that, so a test run with no
-tests still returns 0.
+By default, Catch2 test binaries return non-0 exit code if no tests were run,
+e.g. if the binary was compiled with no tests, the provided test spec matched no
+tests, or all tests [were skipped at runtime](skipping.md#top). This flag
+overrides that, so a test run with no tests still returns 0.
 
 ## Output verbosity
 ```
diff --git a/docs/deprecations.md b/docs/deprecations.md
index 2c9bf551..5926f2c5 100644
--- a/docs/deprecations.md
+++ b/docs/deprecations.md
@@ -26,6 +26,15 @@ to accurately probe the environment for this information so the flag
 where it will export `BAZEL_TEST=1` for purposes like the above. Catch2
 will now instead inspect the environment instead of relying on build configuration.
 
+### `IEventLister::skipTest( TestCaseInfo const& testInfo )`
+
+This event (including implementations in derived classes such as `ReporterBase`)
+is deprecated and will be removed in the next major release. It is currently
+invoked for all test cases that are not going to be executed due to the test run
+being aborted (when using `--abort` or `--abortx`). It is however
+**NOT** invoked for test cases that are [explicitly skipped using the `SKIP`
+macro](skipping.md#top).
+
 ---
 
 [Home](Readme.md#top)
diff --git a/docs/skipping.md b/docs/skipping.md
new file mode 100644
index 00000000..edd2a1a1
--- /dev/null
+++ b/docs/skipping.md
@@ -0,0 +1,72 @@
+<a id="top"></a>
+# Skipping Test Cases at Runtime
+
+> [Introduced](https://github.com/catchorg/Catch2/pull/2360) in Catch2 X.Y.Z.
+
+In some situations it may not be possible to meaningfully execute a test case, for example when the system under test is missing certain hardware capabilities.
+If the required conditions can only be determined at runtime, it often doesn't make sense to consider such a test case as either passed or failed, because it simply can not run at all.
+To properly express such scenarios, Catch2 allows to explicitly _skip_ test cases, using the `SKIP` macro:
+
+**SKIP(** _message expression_ **)**
+
+Example usage:
+
+```c++
+TEST_CASE("copy files between drives") {
+    if(getNumberOfHardDrives() < 2) {
+        SKIP("at least two hard drives required");
+    }
+    // ...
+}
+```
+
+This test case is then reported as _skipped_ instead of _passed_ or _failed_.
+
+The `SKIP` macro behaves similarly to an explicit [`FAIL`](logging.md#top), in that it is the last expression that will be executed:
+
+```c++
+TEST_CASE("my test") {
+    printf("foo");
+    SKIP();
+    printf("bar"); // not printed
+}
+```
+
+However a failed assertion _before_ a `SKIP` still causes the entire test case to fail:
+
+```c++
+TEST_CASE("failing test") {
+    CHECK(1 == 2);
+    SKIP();
+}
+```
+
+## Interaction with Sections and Generators
+
+Sections, nested sections as well as test cases with [generators](generators.md#top) can all be individually skipped, with the rest executing as usual:
+
+```c++
+TEST_CASE("complex test case") {
+  int value = GENERATE(2, 4, 6);
+  SECTION("a") {
+    SECTION("a1") { CHECK(value < 8); }
+    SECTION("a2") {
+      if (value == 4) {
+        SKIP();
+      }
+      CHECK(value % 2 == 0);
+    }
+  }
+}
+```
+
+This test case will report 5 passing assertions; one for each of the three values in section `a1`, as well as one for each in `a2`, except for when `value == 4`.
+
+Note that as soon as one section is skipped, the entire test case will be reported as _skipped_ (unless there is a failing assertion, in which case it will be reported as _failed_ instead).
+
+If all test cases in a run are skipped, Catch2 returns a non-zero exit code by default.
+This can be overridden using the [--allow-running-no-tests](command-line.md#no-tests-override) flag.
+
+---
+
+[Home](Readme.md#top)