Fix compilation (#20028)

Fixed compilation, forgot that var is only available if TESTING is
defined

code/unit_tests/map_tests.dm

@@ -349,12 +349,21 @@
 /datum/unit_test/map_test/no_dirty_vars/start_test()
 	var/test_status
 
+#if defined(TESTING)
+
 	if(length(GLOB.dirty_vars))
 		test_status = TEST_FAIL("There are dirty vars in the map! Read the logs above!")
 		TEST_DEBUG(json_encode(GLOB.dirty_vars))
 	else
 		test_status = TEST_PASS("No dirty vars in the map.")
 
+#else
+
+	test_status = TEST_FAIL("This test was run without the TESTING define set, which isn't supported")
+
+#endif
+
+
 	return test_status
 
 #undef SUCCESS