Add DISCOVERY_MODE to catch_discover_tests

[reddwarf69]

catch_discover_tests is modelled after CMake's gtest_discover_tests, but it's not up to date. Since CMake 3.18 gtest_discover_tests has supported a DISCOVERY_MODE option, which when set to PRE_TEST allows for the test discovery to be delayed until just prior to test execution. This is useful when cross-compiling, since the build machine may be unable to run the test executable. Adding DISCOVERY_MODE to catch_discover_tests would be equally useful.