gh-128657: Skip test_get_builtin_constructor when running with --parallel-threads

[colesbury]

The test modifies sys.modules and is not thread-safe with --parallel-threads.

• Issue: Race in py_digest_by_name, _hashopenssl.c under free-threading #128657

[colesbury]

Test failure seen here:

https://github.com/python/cpython/actions/runs/13272459564/job/37054979703

test test_hashlib failed -- Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython/Lib/test/test_hashlib.py", line 278, in test_get_builtin_constructor
    constructor = get_builtin_constructor('md5')
  File "/home/runner/work/cpython/cpython/Lib/hashlib.py", line 123, in __get_builtin_constructor
    raise ValueError('unsupported hash type ' + name)
ValueError: unsupported hash type md5