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 <yanray.wang@arm.com>

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

tests/scripts/check_test_cases.py

@@ -116,18 +116,13 @@ state may override this method.
         """Iterate over the test cases compat.sh with a similar format."""
         descriptions = self.new_per_file_state() # pylint: disable=assignment-from-none
         compat_cmd = ['sh', file_name, '--list-test-case']
-        result = subprocess.run(compat_cmd,
-                                stdout=subprocess.PIPE,
-                                check=False)
-        if result.returncode != 0:
-            print(*compat_cmd, 'returned', str(result.returncode))
-            return
-        else:
-            # Assume compat.sh is responsible for printing identical format of
-            # test case description between --list-test-case and its OUTCOME.CSV
-            description = result.stdout.strip().split(b'\n')
-            for idx, descrip in enumerate(description):
-                self.process_test_case(descriptions, file_name, idx, descrip)
+        compat_output = subprocess.check_output(compat_cmd,
+                                                stderr=subprocess.STDOUT)
+        # Assume compat.sh is responsible for printing identical format of
+        # test case description between --list-test-case and its OUTCOME.CSV
+        description = compat_output.strip().split(b'\n')
+        for idx, descrip in enumerate(description):
+            self.process_test_case(descriptions, file_name, idx, descrip)
 
     @staticmethod
     def collect_test_directories():