Catch.cmake: Add new DISCOVERY_MODE option to catch_discover_tests

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

docs/cmake-integration.md

@@ -126,6 +126,7 @@ catch_discover_tests(target
                      [OUTPUT_DIR dir]
                      [OUTPUT_PREFIX prefix]
                      [OUTPUT_SUFFIX suffix]
+                     [DISCOVERY_MODE <POST_BUILD|PRE_TEST>]
 )
 ```
 
@@ -198,6 +199,16 @@ If specified, `suffix` is added to each output file name, like so
 `--out dir/<test_name>suffix`. This can be used to add a file extension to
 the output file name e.g. ".xml".
 
+* `DISCOVERY_MODE mode`
+
+If specified allows control over when test discovery is performed.
+For a value of `POST_BUILD` (default) test discovery is performed at build time.
+For a a value of `PRE_TEST` test discovery is delayed until just prior to test
+execution (useful e.g. in cross-compilation environments).
+``DISCOVERY_MODE`` defaults to the value of the
+``CMAKE_CATCH_DISCOVER_TESTS_DISCOVERY_MODE`` variable if it is not passed when
+calling ``catch_discover_tests``. This provides a mechanism for globally
+selecting a preferred test discovery behavior.
 
 ### `ParseAndAddCatchTests.cmake`