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