test: check argc in symbol-test

Check argc mainly to fix unused parameter warning:

test/symbol-test.cc:7:14: warning: unused parameter ‘argc’ [-Wunused-parameter]
  int main(int argc, char** argv) {
               ^

This makes more sense than telling the compiler it's unused on
purpose.

test/symbol-test.cc

@@ -8,6 +8,9 @@ int main (int argc, char** argv)
     notmuch_database_t *notmuch;
     char *message = NULL;
 
+    if (argc != 3)
+	return 1;
+
     if (notmuch_database_open_verbose (argv[1], NOTMUCH_DATABASE_MODE_READ_ONLY,
 				       &notmuch, &message)) {
 	if (message) {