Tests using `benchmark` fixture from `pytest-benchmark` should be marked as unsafe

[ngoldbaum]

Currently this is what happens if you run a test using the fixture:

goldbaum at Nathans-MBP in ~/Documents/cryptography on free-threaded-support!
(tests-parallel) ± pytest --parallel-threads=4 -svx --pdb
============================= test session starts ==============================
platform darwin -- Python 3.14.0rc1, pytest-8.4.1, pluggy-1.6.0 -- /Users/goldbaum/Documents/cryptography/.nox/tests-parallel/bin/python3
cachedir: .pytest_cache
benchmark: 5.1.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
OpenSSL: OpenSSL 3.5.1 1 Jul 2025
FIPS Enabled: False
rootdir: /Users/goldbaum/Documents/cryptography
configfile: pyproject.toml
plugins: run-parallel-0.5.0, xdist-3.8.0, cov-6.2.1, benchmark-5.1.0
collected 3546 items
Collected 3487 items to run in parallel

tests/bench/test_aead.py::test_chacha20poly1305_encrypt PARALLEL FAILED  [  0%]
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> traceback >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

benchmark = <pytest_benchmark.fixture.BenchmarkFixture object at 0x11d6d2f90>

    @pytest.mark.skipif(
        not _aead_supported(ChaCha20Poly1305),
        reason="Requires OpenSSL with ChaCha20Poly1305 support",
    )
    def test_chacha20poly1305_encrypt(benchmark):
        chacha = ChaCha20Poly1305(b"\x00" * 32)
>       benchmark(chacha.encrypt, b"\x00" * 12, b"hello world plaintext", b"")

tests/bench/test_aead.py:31:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <pytest_benchmark.fixture.BenchmarkFixture object at 0x11d6d2f90>
function_to_benchmark = <built-in method encrypt of cryptography.hazmat.bindings._rust.openssl.aead.ChaCha20Poly1305 object at 0x11bea0750>
args = (b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00', b'hello world plaintext', b'')
kwargs = {}

    def __call__(self, function_to_benchmark, *args, **kwargs):
        if self._mode:
            self.has_error = True
>           raise FixtureAlreadyUsed(f'Fixture can only be used once. Previously it was used in {self._mode} mode.')
E           pytest_benchmark.fixture.FixtureAlreadyUsed: Fixture can only be used once. Previously it was used in benchmark(...) mode.

.nox/tests-parallel/lib/python3.14/site-packages/pytest_benchmark/fixture.py:153: FixtureAlreadyUsed
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> entering PDB >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> PDB post_mortem >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> /Users/goldbaum/Documents/cryptography/.nox/tests-parallel/lib/python3.14/site-packages/pytest_benchmark/fixture.py(153)__call__()
-> raise FixtureAlreadyUsed(f'Fixture can only be used once. Previously it was used in {self._mode} mode.')

I tried the simplest thing of just adding "benchmark" to THREAD_UNSAFE_FIXTURES and that didn't seem to work.