Don't ignore errors raised by `PyErr_WarnFormat` in `warn_about_fork_with_threads`
Instead, ignore the warnings in all test code that forks. (That's a lot of functions.)
In `test_support`, make `ignore_warnings` a context manager (as well as decorator),
and add a `message` argument to it.
Also add a `ignore_fork_in_thread_deprecation_warnings` helper for the deadlock-in-fork
warning.
