bpo-44430: Refactor `sqlite3` threading tests (GH-26748)

1. Rewrite ThreadTests with a _run_test() helper method that does the heavy lifting
2. Add test.support.threading_helper.reap_threads to _run_test()
3. Use _run_test() in all threading tests
4. Add test case for sqlite3.Connection.set_trace_callback
5. Add test case for sqlite3.Connection.create_collation