chore(python): Noxfile recognizes that tests can live in a folder (#262)

Source-Link: googleapis/synthtool@4760d8d
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:f0e4b51deef56bed74d3e2359c583fc104a8d6367da3984fc5c66938db738828

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

documentai/snippets/noxfile.py

@@ -187,6 +187,7 @@ def _session_tests(
 ) -> None:
     # check for presence of tests
     test_list = glob.glob("*_test.py") + glob.glob("test_*.py")
+    test_list.extend(glob.glob("tests"))
     if len(test_list) == 0:
         print("No tests found, skipping directory.")
     else: