[mypyc] Add test for incremental builtin_base class construction across groups

[georgesittas]

The fix for this was included in #21369, but no dedicated test was added.

This adds testIncrementalBuiltinBaseClassConstruction to run-multimodule.test: three modules compiled with separate=True, where step 2 changes a helper module's signature to force the caller to be recompiled while the exception module is only loaded from cache.

[p-sawicki]

can i get instructions how to reproduce the original issue? i checked out the parent of the merge commit of #21369 and cherry-picked this test but pytest mypyc/test/test_run.py::TestRunSeparate::run-multimodule.test::testIncrementalBuiltinBaseClassConstruction_separate passed. i've also patched fudge_dir_mtimes in case it forced recompilation without it but it still passed.

[georgesittas]

Ah, it seems like the test framework only treats files matching other*.py as additional compiled sources (source, comment), anything else is written to disk as plain Python, not added to the build.

I missed this and named the helper files myerrors.py and myutil.py, so MyError would not be in a compiled group. Which is why the bug, which needs cross-group emit, is not triggered.

Sorry for the confusion. Feel free to give it another shot; I also verified that the test fails in the parent commit and should pass in this PR.