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>

tests/ExtraTests/CMakeLists.txt

@@ -488,15 +488,32 @@ set_tests_properties(TestSpecs::EmptySpecWithNoTestsFails
   PROPERTIES
     WILL_FAIL ON
 )
+
 add_test(
   NAME TestSpecs::OverrideFailureWithEmptySpec
   COMMAND $<TARGET_FILE:NoTests> --allow-running-no-tests
 )
+
 add_test(
   NAME List::Listeners::WorksWithoutRegisteredListeners
   COMMAND $<TARGET_FILE:NoTests> --list-listeners
 )
+
+
+add_executable(AllSkipped ${TESTS_DIR}/X93-AllSkipped.cpp)
+target_link_libraries(AllSkipped PRIVATE Catch2::Catch2WithMain)
+
+add_test(
+  NAME TestSpecs::SkippingAllTestsFails
+  COMMAND $<TARGET_FILE:AllSkipped>
+)
+set_tests_properties(TestSpecs::SkippingAllTestsFails
+  PROPERTIES
+    WILL_FAIL ON
+)
+
 set( EXTRA_TEST_BINARIES
+    AllSkipped
     PrefixedMacros
     DisabledMacros
     DisabledExceptions-DefaultHandler