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>

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).
+
 ---
 
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