Add a scoped_env utility for tests

[zcbenz]

Replace a common pattern in tests:

        name = "MLX_SDPA_PAD_HEAD_DIM"
        previous = os.environ.get(name)
        try:
            os.environ[name] = "1"
            run_tests()
        finally:
            if previous is None:
                os.environ.pop(name, None)
            else:
                os.environ[name] = previous

With a utility:

        with mlx_tests.scoped_env(MLX_SDPA_PAD_HEAD_DIM=1):
            run_tests()

[nastya236]

Thank you! Very nice refactoring!