chore: revert SUB_DIRECTORY in favor of TEST_DIRECTORIES env var (GoogleCloudPlatform#1335)

Author: Ace Nassri

.kokoro/system_tests.sh

@@ -59,11 +59,5 @@ fi
 # Install global test dependencies
 composer install -d testing/
 
-# cd into specific subdirectory (if appropriate)
-MAIN_DIR=$(pwd)
-if [ ! -z "${SUB_DIRECTORY}" ]; then
-  cd $SUB_DIRECTORY
-fi
-
 # Run tests
-bash $MAIN_DIR/testing/run_test_suite.sh
+bash testing/run_test_suite.sh