tester: return 1 if at least one test has failed

src/tester.c

@@ -236,4 +236,6 @@ int main(int argc, char **argv)
 
     printf("\nSummary: %d passed, %d failed\n", success_tests_counter, failed_tests_counter);
     close(f);
+
+    return !(!failed_tests_counter);
 }