rc_update: switch back to protected scope and use fixture for testing

src/modules/rc_update/rc_update.h

@@ -89,6 +89,7 @@ public:
 
 	int print_status() override;
 
+protected:
 	static constexpr uint64_t VALID_DATA_MIN_INTERVAL_US{1_s / 3}; // assume valid RC input is at least 3 Hz
 
 	void Run() override;