You don’t need to post on the forums to be a Tester, though there are often ‘Secret Testers’ that get posted here, so it’s worth checking in regularly so you can pop your name down if you see one you like. (there’s more info about them in the links too)
Welcome to the forum @MrTommy5 , as stated above no, for the main tester list you certainly don’t have to be a regular on the forum you just have to write good and helpful reviews of products you already own.
Some testers, particularly secret ones however, are announced on the forum and you would need to post a reply to show interest in these.