Fix tests_api submodule initialization for CI tests

The tests_api submodule (containing validate_response module) was not
being initialized in CI, causing all Tavern API tests to fail with
"ModuleNotFoundError: No module named 'validate_response'".

This happened because:
1. GIT_SUBMODULE_STRATEGY is set to 'none' (manual init required)
2. The .init_submodules script only initialized haf, reputation_tracker,
   and hafah - but not tests_api
3. The recent switch to GIT_STRATEGY: fetch meant existing checkouts
   were reused without the missing submodule content

Fix:
- Add tests_api to the .init_submodules script for consistency
- Add inline tests_api initialization to e2e_benchmark_on_postgrest job