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>

src/catch2/internal/catch_assertion_handler.cpp

@@ -50,6 +50,13 @@ namespace Catch {
         if (m_reaction.shouldThrow) {
             throw_test_failure_exception();
         }
+        if ( m_reaction.shouldSkip ) {
+#if !defined( CATCH_CONFIG_DISABLE_EXCEPTIONS )
+            throw Catch::TestSkipException();
+#else
+            CATCH_ERROR( "Explicitly skipping tests during runtime requires exceptions" );
+#endif
+        }
     }
     void AssertionHandler::setCompleted() {
         m_completed = true;