[mypyc] Add test for incremental builtin_base class construction across groups (#21524)

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.