Add DISCOVERY_MODE to doctest_discover_tests

[bilderbuchi]

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 doctest_discover_tests would be equally useful. ❤️

Upstream docs at CMake: https://cmake.org/cmake/help/latest/module/GoogleTest.html#command:gtest_discover_tests
Equivalent issue at Catch2, which is closely related to doctest afaik: catchorg/Catch2#2493
Relevant Catch2 PR implementing that feature: catchorg/Catch2#2670

Also mentioned in here: #645 (comment)
cc @curldivergence