Make `test_save_pretrained_signatures` slow test
# What does this PR do?

`UtilsFunctionsTest.test_save_pretrained_signatures` in `tests/test_modeling_tf_common.py` is introduced on Oct. 2022, which runs about ~60 seconds, and failed a lot of times **on Push CI** due to the timeout limit on Push CI job setting (`PYTEST_TIMEOUT`).

Notice that, on CircleCI, we have `PYTEST_TIMEOUT=120` - as we use flag `-n 8` to run tests in parallel. I don't want to set `120` for Push CI at this moment (without running with it first), as it might make the CI slow down (a lot).
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks! I'd like to have @Rocketknight1 and @gante approve on this before we merge (or find a way to make this test slower).