Fixes #2615

* 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>

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

Closes #2132

This fixes multiple issues with random generators, with the most
important one being that multiple nested generators could return
values from the same sequence, due to internal implementation
details of `GENERATE`, and how they interact with test case
paths.

The cost of doing this is that given this simple `TEST_CASE`,
```cpp
TEST_CASE("foo") {
    auto i = GENERATE(take(10, random(0, 100));
    SECTION("A") {
        auto j = GENERATE(take(10, random(0, 100));
    }
    SECTION("B") {
        auto k = GENERATE(take(10, random(0, 100));
    }
}
```

`k` will have different values between running the test as
a whole, e.g. with `./tests "foo"`, and running only the "B"
section with `./tests "foo" -c "B"`.

I consider this an acceptable cost, because the only alternative
would be very messy to implement, and add a lot of brittle and
complex code for relatively little benefit.

If this calculation changes, we will need to instead walk
the current tracker tree whenever a random generator is being
constructed, check for random generators on the path to root,
and take a seed from them.

We no longer use `std::rand` anywhere, so there is no point,
and it might even interfere with our users use of `std::(s)rand`.

This might become potentially useful in the future, when we want
to provide the ability to forward jump generators, to be able to
simply reproduce specific input to a test.

I am not yet sure how it would work, but the basic idea is that
it could be similar to the `-c` switch for selecting specific
`SECTION` in a test case.