Add a basic test suite

[JesseTG]

This PR adds a test suite that is powered by CTest. It is adapted from the test suite for melonDS DS.

All you have to do is write a program that returns 0 upon success, or something else upon failure. No test suite library is necessary.

It does not add any new dependencies except that the test utility library requires C++20 for std::counting_semaphore (as I didn't want to pull in Qt or add a third-party library).

This PR is not intended to do any of the following:

• Add comprehensive tests. It will add some basic ones that exercise the test suite's features plus documentation on how to write more tests.
• Provide audio or video output. Future tests are welcome to do this with something like SDL. You can still retrieve audio and video from NDS instances and use them yourself.
• Test the UI. There's no inherent reason that couldn't be done, but you'd need a way to fake input. That may come directly by feeding simulated input, or it may come by triggering events manually in sequence.

There are still a few things I'd like to add before it's ready for merge:

• Integrate this into the GitHub pipeline so that the build fails if any test fails
• Add a few that use ROMs and/or system files
• Copy any provided ROMs and system files into a temporary directory so that tests don't clobber them
• Write documentation on adding tests, and on running the test suite locally