chore(python): Noxfile recognizes that tests can live in a folder (#1334)

Pytest works just fine with tests organized inside a `tests` folder. I see no reason why we shouldn't accept that. Also, I wanted to put tests in dedicated folder in my samples ;) Tested it locally

Author: m-strzelczyk

synthtool/gcp/templates/python_samples/noxfile.py.j2

@@ -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: