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/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
 ```