check_test_cases.py: use check_output to capture error and return

This commit includes:
 - use subprocess.check_output to report error and capture return
   value
 - add comment as a reminder for option --list-test-case

Signed-off-by: Yanray Wang <[email protected]>

tests/compat.sh

@@ -121,6 +121,7 @@ print_test_case() {
     done
 }
 
+# list_test_case lists all potential test cases in compat.sh without execution
 list_test_case() {
     reset_ciphersuites
     for TYPE in $TYPES; do
@@ -169,6 +170,8 @@ get_options() {
             -M|--memcheck)
                 MEMCHECK=1
                 ;;
+            # Please check scripts/check_test_cases.py correspondingly
+            # if you have to modify option, --list-test-case
             --list-test-case)
                 list_test_case
                 exit 0