tc-testing: fix the -l argument bug in tdc.py

This patch fixes a bug in the tdc script, where executing tdc
with the -l argument would cause the tests to start running
as opposed to listing all the known test cases.
Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/tc-testing/tdc.py

@@ -49,7 +49,7 @@ def exec_cmd(command, nsonly=True):
         stderr=subprocess.PIPE)
     (rawout, serr) = proc.communicate()
 
-    if proc.returncode != 0:
+    if proc.returncode != 0 and len(serr) > 0:
         foutput = serr.decode("utf-8")
     else:
         foutput = rawout.decode("utf-8")
@@ -203,7 +203,7 @@ def set_args(parser):
                         help='Run tests only from the specified category, or if no category is specified, list known categories.')
     parser.add_argument('-f', '--file', type=str,
                         help='Run tests from the specified file')
-    parser.add_argument('-l', '--list', type=str, nargs='?', const="", metavar='CATEGORY',
+    parser.add_argument('-l', '--list', type=str, nargs='?', const="++", metavar='CATEGORY',
                         help='List all test cases, or those only within the specified category')
     parser.add_argument('-s', '--show', type=str, nargs=1, metavar='ID', dest='showID',
                         help='Display the test case with specified id')
@@ -357,10 +357,10 @@ def set_operation_mode(args):
     testcases = get_categorized_testlist(alltests, ucat)
 
     if args.list:
-        if (len(args.list) == 0):
+        if (args.list == "++"):
             list_test_cases(alltests)
             exit(0)
-        elif(len(args.list > 0)):
+        elif(len(args.list) > 0):
             if (args.list not in ucat):
                 print("Unknown category " + args.list)
                 print("Available categories:")