register.h: Add internal_flag to Pass

Update experimental pass warnings to use a shared function.  Reduces repetition, and also allows all of the warning flags to be combined (which at present is just experimental and the new internal).
Update `test_*` passes to call `internal()` in their constructors.

kernel/register.h

@@ -69,11 +69,16 @@ struct Pass
 	int call_counter;
 	int64_t runtime_ns;
 	bool experimental_flag = false;
+	bool internal_flag = false;
 
 	void experimental() {
 		experimental_flag = true;
 	}
 
+	void internal() {
+		internal_flag = true;
+	}
+
 	struct pre_post_exec_state_t {
 		Pass *parent_pass;
 		int64_t begin_ns;