Renamed internal runner field filter -> if_

This makes it more consistent with the actual test field, at the cost of
the symbol collision.

runners/test_runner.h

@@ -52,7 +52,7 @@ struct test_case {
     const test_define_t *defines;
     size_t permutations;
 
-    bool (*filter)(void);
+    bool (*if_)(void);
     void (*run)(struct lfs_config *cfg);
 };