Chore: Decrease backend test suite time (a little) (#14213)

* Chore: Speed up test setup by hashing passwords with MD5 and batching index writes

Don't use Django's default PBKDF2, about 600 ms per use and 110 uses across the suite. Switches to MD5 instead.

Also fixes a test that didn't batch update the search index

* Chore: Stop the invalid webhook params test from waiting on a Celery broker

test_workflow_webhook_action_url_invalid_params_headers left send_webhook.apply_async unpatched, so it tried to actually enqueu and waited for the timeout.

src/conftest.py

@@ -31,6 +31,18 @@ def faker_session_locale() -> str:
     return "en_US"
 
 
+@pytest.fixture(autouse=True)
+def _fast_password_hasher(settings: Settings) -> None:
+    """Hash test passwords with MD5 instead of Django's default PBKDF2.
+
+    PBKDF2 is deliberately slow, and every ``admin_user`` or
+    ``create_superuser`` call pays for it: about 600 ms each. No test depends
+    on the hash format, only on ``check_password`` and on the stored value
+    changing when the password does.
+    """
+    settings.PASSWORD_HASHERS = ["django.contrib.auth.hashers.MD5PasswordHasher"]
+
+
 @pytest.fixture(autouse=True)
 def _clear_content_type_caches() -> None:
     """Clear Django's ContentType cache and guardian's lru_cache before each test.