Having tested several games in my time, from open betas of major releases to alpha testing for community mods, let me give you an advice:
IT ISN'T FUN
Okay, open testing is, because it's pretty much just playing the game for free for a few weeks before the launch. But it's nothing you'd get paid for.
But any more in depth of game testing is very tedious. All you do is basically look for bugs and exploits and try to reproduce them. That means doing the same things ad nauseam, and protocoll them as detailed as possible. And for every single bug, you have to do it everytime a fix tried to deal with it.
And afterwards, you can't really enjoy the final product, as firstly, it got ruined because of the repetition (much like a word loses it's meaning if you repeat it numerous times), and because you know of every bug that still remained in it, and no matter how well polished, they will still be everywhere, and to you, be very obvious.
So honestly, if you love games, DON'T become a tester, rather become a reviewer. But that's also a hard enough feat, as there are millions of people wanting to be one, so it's nigh impossible to get a job, even if you have a profound journalistic background.