Provide the certificate file mocked by the generated mTLS tests.

--- /dev/null
+++ tests/unit/gapic/conftest.py
@@ -0,0 +1,11 @@
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def certificate_config_file():
+    # The generated mTLS tests mock open(); google-auth also checks existence.
+    config = Path("mock_certificate_config.json")
+    config.write_text("{}")
+    return config
