This target previously did not specify its minimum C++ standard.
AppleClang was emitting warnings about the use of C++11 features
which is fixed by telling CMake that this target requires C++14.
Because this target does not link to the existing CMake targets
it never inherited that C++ standard requirement.

This is not necessary so we can remove it. Most conditionals already
omit this anyways.

Closes #2678

* 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>

This changes the compact reporter's summary of test run totals to use
the same format as the console reporter. This means that while output is
no longer on a single line (two instead), it now includes totals for
`failedButOk` test cases and assertions, which were previously missing.

Closes #2491

Closes #2490