tests: clean up properly after completion (#179)

Author: daniel-sanche

.kokoro/environment_tests.sh

@@ -65,7 +65,7 @@ TEST_STATUS_CODE=$?
 
 # destroy resources
 echo "cleaning up..."
-/workspace/python-logging/tests/environment/envctl/envctl python $ENVIRONMENT destroy
+${PROJECT_ROOT}/tests/environment/envctl/envctl python $ENVIRONMENT destroy
 
 # exit with proper status code
 exit $TEST_STATUS_CODE