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/catch_session.cpp

@@ -341,6 +341,12 @@ namespace Catch {
                 return 2;
             }
 
+            if ( totals.testCases.total() > 0 &&
+                 totals.testCases.total() == totals.testCases.skipped
+                && !m_config->zeroTestsCountAsSuccess() ) {
+                return 4;
+            }
+
             // Note that on unices only the lower 8 bits are usually used, clamping
             // the return value to 255 prevents false negative when some multiple
             // of 256 tests has failed