Add an easy way to skip tests due to missing dependencies

In libfprint I have tests that require pulling in additional dependencies. Right now I am adding the following code:

https://gitlab.freedesktop.org/libfprint/libfprint/commit/19f239ce617ffaf965871696167d0f767a0d6cea#af2b927c2ec3dd69e8b5addb910b2d1c766bea76_72_88

i.e. I print a warning and create a dummy test that just always skip and then avoid building the executable and test.

The logic seems unnecessarily complicated. I could imagine a "skip" parameter for test, to just skip (ignoring all other parameters). Though in this case I would also need to skip building the corresponding C file, which could be a bit uglier again.

Review the linked libfprint commit and Meson's existing test and build-parameter handling first. Define how a test-level skip should behave when dependencies are missing, including whether its corresponding C source is built; done means dependency-dependent tests can be skipped without dummy tests or unnecessary executables.

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