Introducing a new DISCOVERY_MODE mode option, which provides greater
control over when catch_discover_tests perforsm test discovery.
It has two supported modes:
* POST_BUILD: The default behavior, which adds a POST_BUILD command
to perform test discovery after the test has been built as was
always done so far.
* PRE_TEST: New mode, which delays test discovery until test execution.
The generated include file generates the appropriate CTest files at
runtime and regenerates the CTest files when the executable is
updated.
This mode can be used in build-environments that don't allow for
executing the linked binaries at build-time (like in a
cross-compilation environment).
DISCOVERY_MODE can be controlled in two ways:
1. Setting the DISCOVERY_MODE when calling catch_discover_tests.
2. Setting the global CMAKE_CATCH_DISCOVER_TESTS_DISCOVERY_MODE prior
to calling gtest_discover_tests.
Closes #2493