test(gcp): Only run GCP tests when they should

GCP tests have been running in our common test suite, including on Python versions other than 3.7 (the only version which supports the GCP integration), even though we have a separate `py3.7-gcp` tox environment for these tests. The tests take a long time, so only executing in the appropriate `tox` environment should speed up CI time.

Author: szokeasaurusrex

tests/integrations/gcp/__init__.py

@@ -0,0 +1,6 @@
+import pytest
+import os
+
+
+if "gcp" not in os.environ.get("TOX_ENV_NAME", ""):
+    pytest.skip("GCP tests only run in GCP environment", allow_module_level=True)