test: add intentional unit test failure to verify CI detection

tests/unittests/test_common.c

@@ -12,7 +12,7 @@ replace_one_substr(void** state)
 
     char* result = str_replace(string, sub, new);
 
-    assert_string_equal("it was a string", result);
+    assert_string_equal("INTENTIONAL FAILURE", result);
 
     free(result);
 }