Generate: smaller TF serving test
# What does this PR do?

`tests/test_modeling_tf_common.py::UtilsFunctionsTest::test_generate_tf_function_export` is often failing because it times out (>60s).

The previous version took ~35s in my machine. This PR's takes ~19s, which may avoid the time out issue.

If it still fails, a custom config must be added :)
_The documentation is not available anymore as the PR was closed or merged._