Add CMake module to discover tests

[bracketttc]

Adapted shamelessly from the official Google Test module distributed
with CMake 3.9 and higher.

[bracketttc]

• Add to README.md

[silentbicycle]

Rather than scanning the test source, it might be more reliable to use the test runner's -l flag, which lists the suite and test case names in a consistent and easily scraped format. That does require building and executing the program, but it doesn't actually run any of the tests.

It would probably be good to check if running the same test case in a loop with different arguments (such as gradually increasing bounds) is handled well. greatest_set_test_suffix can be used to add a parameter suffix to the test case names, which would otherwise be the same each time.

(Also, is "adapting shamelessly" okay with the Google Test module's licensing?)

I don't see anything problematic here, but I don't know CMake very well enough to review the code in depth.

[bracketttc]

The GoogleTest module has functions for both, so that shouldn't be hard. Not 100% sure why I only took one of the functions except that it does what I want and the other sometimes has issues with static initialization (if you write test executables with static initialization that can fail, your build system can fail).

CMake's modules are under BSD 3 clause, so I think it's cool to lift, but should probably reproduce the license in this module as well.

https://github.com/Kitware/CMake/blob/460e812369db398da34e14c9d40c3ecec06afaf1/Modules/GoogleTest.cmake