chore: detect samples tests in nested directories (#456)

Source-Link: googleapis/synthtool@50db768
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:e09366bdf0fd9c8976592988390b24d53583dd9f002d476934da43725adbb978

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

Author: gcf-owl-bot[bot]

Dialogflow-CX/noxfile.py

@@ -207,8 +207,8 @@ def _session_tests(
     session: nox.sessions.Session, post_install: Callable = None
 ) -> None:
     # check for presence of tests
-    test_list = glob.glob("*_test.py") + glob.glob("test_*.py")
-    test_list.extend(glob.glob("tests"))
+    test_list = glob.glob("**/*_test.py", recursive=True) + glob.glob("**/test_*.py", recursive=True)
+    test_list.extend(glob.glob("**/tests", recursive=True))
 
     if len(test_list) == 0:
         print("No tests found, skipping directory.")