GDScript: Allow tests to run on release builds

- Fix compilation issues by disabling warnings on release builds. This
  also strips warnings from expected result before the comparison to
  avoid false mismatches.
- Add a `#debug-only` flag to tests. Must be the first line of the test
  script. Those won't run with release builds. Can be used for test
  cases that rely on checks only available on debug builds.

modules/gdscript/tests/scripts/runtime/errors/callable_call_after_free_object.gd

@@ -1,3 +1,4 @@
+#debug-only
 func test():
 	var node := Node.new()
 	var inside_tree = node.is_inside_tree

modules/gdscript/tests/scripts/runtime/errors/callable_call_after_free_object.out

@@ -2,5 +2,5 @@ GDTEST_RUNTIME_ERROR
 >> SCRIPT ERROR
 >> on function: test()
 >> runtime/errors/callable_call_after_free_object.gd
->> 5
+>> 6
 >> Attempt to call function 'null::is_inside_tree (Callable)' on a null instance.