Adds file to run all tests in PR (GoogleCloudPlatform#772)

Author: bshaffer

testing/RUN_ALL_TESTS

@@ -0,0 +1,2 @@
+# Modify this file with whitespace in a PR in order to run all tests for the PR.
+# See "testing/run_test_suite.sh" for more details.

testing/run_test_suite.sh

@@ -64,11 +64,9 @@ FAILED_FLAKY_FILE=${TMP_REPORT_DIR}/failed_flaky
 # (will be empty if running from "master").
 FILES_CHANGED=$(git diff --name-only HEAD $(git merge-base HEAD master))
 
-# If any files outside the sample directories changed, or if we are not
-# on a Pull Request, run the whole test suite.
-if grep -q ^testing\/ <<< "$FILES_CHANGED" || \
-   grep -v ^.kokoro\/secrets.sh.enc$ <<< "$FILES_CHANGED" | grep -q ^.kokoro\/ || \
-    grep -qv \/ <<< "$FILES_CHANGED" || \
+# If the file RUN_ALL_TESTS is modified, or if we were not triggered from a Pull
+# Request, run the whole test suite.
+if grep -q ^testing\/RUN_ALL_TESTS$ <<< "$FILES_CHANGED" || \
     [ -z "$IS_PULL_REQUEST" ]; then
     RUN_ALL_TESTS=1
 else