test(marktree): expose test functions in release builds

In order to run the marktree unit test in release mode, the test functions need to be available even when NDEBUG is defined.

Keep the body of marktree_check a nop during release builds, which limits the usefulness of the testing, but at least lets the tests run.

src/nvim/marktree.c

@@ -2330,7 +2330,6 @@ void marktree_check(MarkTree *b)
 #endif
 }
 
-#ifndef NDEBUG
 size_t marktree_check_node(MarkTree *b, MTNode *x, MTPos *last, bool *last_right,
                            const uint32_t *meta_node_ref)
 {
@@ -2485,8 +2484,6 @@ bool mt_recurse_nodes_compare(MTNode *x, PMap(ptr_t) *checked)
   return true;
 }
 
-#endif
-
 // TODO(bfredl): kv_print
 #define GA_PUT(x) ga_concat(ga, (char *)(x))
 #define GA_PRINT(fmt, ...) snprintf(buf, sizeof(buf), fmt, __VA_ARGS__); \