updated ads template tests

Author: daniel-sanche

gapic/ads-templates/tests/unit/gapic/%name_%version/%sub/test_%service.py.j2

@@ -94,13 +94,15 @@ def test__get_default_mtls_endpoint():
     sandbox_endpoint = "example.sandbox.googleapis.com"
     sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
     non_googleapi = "api.example.com"
+    custom_endpoint = ".custom"
 
     assert {{ service.client_name }}._get_default_mtls_endpoint(None) is None
     assert {{ service.client_name }}._get_default_mtls_endpoint(api_endpoint) == api_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(api_mtls_endpoint) == api_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_endpoint) == sandbox_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_mtls_endpoint) == sandbox_mtls_endpoint
     assert {{ service.client_name }}._get_default_mtls_endpoint(non_googleapi) == non_googleapi
+    assert {{ service.client_name }}._get_default_mtls_endpoint(custom_endpoint) == custom_endpoint
 
 
 @pytest.mark.parametrize("client_class,transport_name", [