Add note in authoring guide about testing (#2683)

Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>

AUTHORING_GUIDE.md

@@ -413,6 +413,8 @@ if __name__ == '__main__':
   standard- these samples are only be tested against Python 2.7.
 * Samples that use App Engine Standard should use the App Engine testbed for
   system testing. See existing App Engine tests for how to use this.
+* All tests should be independent of one another and should create and tear
+  down resources needed to execute. Using UUIDs to avoid resource collision is recommended.
 
 ## Running tests and automated tools