test: show the tool exit status on failure

We already do so for the non-successful cases, let's do this here too so we know
whether it was a signal or a normal exit.

test/tool-option-parsing.py

@@ -100,7 +100,7 @@ class XkbcliTool:
             for testfunc, reason in self.skipError:
                 if testfunc(rc, stdout, stderr):
                     raise unittest.SkipTest(reason)
-        assert rc == 0, (stdout, stderr)
+        assert rc == 0, (rc, stdout, stderr)
         return stdout, stderr
 
     def run_command_invalid(self, args):