Fix @reap_threads usage in test_free_threading.test_json (#158937)

Use the decorator on test methods, not on the whole class.

Previously, test_json ran no tests:

    $ ./python -m test -v test_free_threading.test_json -v
    ...
    Ran 0 tests in 0.000s

    NO TESTS RAN

Add also a check in @reap_threads to detect misuse (on a class).