fix(tests): two tests fail on free-threaded CPython 3.14t, likely from context-aware warnings #1731

Describe the bug
On free-threaded CPython 3.14t, the experimental leg added by #1725 fails two tests (job 115915976088, run on 296ad6b71):

• tests/protocols/misc/test_pcapng_unit.py::PCAPNGNegativeLengthTests::test_the_captured_len_vector_parses_under_a_memory_cap, failing at :4802 with AssertionError: 1 != 0.
• tests/protocols/test_trial_parse_warnings_1580_unit.py::TestTrialParseWarnings::test_hold_is_local_to_its_context, failing at :392 with Lists differ: [] != ['another context'].

Every other test passes.

Reproduction
Run those two tests on a 3.14t interpreter (setup-python 3.14t).

Expected behavior
Both pass, so the 3.14t leg can be promoted to blocking (#1660, option 1).

Unverified hypothesis: free-threaded builds default sys.flags.context_aware_warnings (and thread_inherit_context) to on. warnings.catch_warnings then records per context. The second test checks which context sees a held warning, and the first counts warnings, so one cause may explain both. A worker should confirm it, or rule it out, before fixing.