oauth: update test runner for per-module split
The concurrent commit 180d7ab11 landed the new per-module test files
(test_client_auth, test_par, test_provider, test_redirect_uri,
test_authorization_url, test_parse_token_response, test_helpers) but
not the runner or the original kitchen sink. Finish the split:
- Remove test_regressions.ml (tests are now in the per-module files).
- Update test.ml to call the per-module suites.