gh-118433: Temporarily skip `test_interrupt_main_subthread` in free-threaded builds (#118485)

Free-threaded builds can intermittently tickle a longstanding bug (24 years!)
in the implementation of `threading.Condition`, leading to flakiness in the
test suite. Fixing the underlying issue will require more discussion, and will
likely apply to most of the concurrency primitives in the `threading` module
that are written in Python. See gh-118433 for more details.