Build cython tests as part of the main test suite

[mdboom]

Fix #1973. It's too easy to forget tests that aren't part of the regular test-running suite.

This automatically builds these tests if precompiled objects aren't found. This allows us to keep compiling on the build machine and testing on a machine without a compiler, while also allowing for more seamless local development.

[leofang]

Apology, @mdboom, for my recollection coming in the late night. It did not click for me until now.

If you don't mind, I would like to propose to kill this effort, because we split it for a very good reason: Having the Cython tests run by default messes up QA tests every single time. We ended up being chased after due to QA's insufficient/imperfect environment issues, as repeatedly discussed in recent team syncs.

A similar discussion can be found in the Oct-Nov 2024 team meeting minutes. We also had a public issue tracking the splitting: #214. Let us not revert the split and take on more unnecessary burdens.

[rwgk]

@leofang I'm not sure if this is relevant here, but I believe QA has been running the cython tests in the 13.2 test cycle, probably also in the 13.1 cycle. For sure, I have them in our reference scripts. Please let me know if finding out for sure (by asking the QA team for confirmation) would change your opinion about this PR.

[leofang]

Please let me know if finding out for sure (by asking the QA team for confirmation) would change your opinion about this PR.

Yes, it'd be nice to know if/how they run the Cython tests in the past few releases of both cuda-bindings and cuda-core.