What if I receive key "for Beta Testing"?

It depends. According to the Guidelines: https://www.steamgifts.com/about/guidelines

"Beta keys which do not provide unrestricted access to the full game in the future, guest passes, demos, and coupons cannot be given away."

So if the keys does provide unrestricted access to the game now and forever, mark it as received. It it does not - for example it is limited in time - then do not mark as received.

Usually there is no problem with "Beta Testing" keys, they should give full access to the game.

But i'm unsure about the rules, as i read something about it recently, but can't remember where, could have been another site

You need to check which version was activated (it's recommended to have SteamDB extension installed in your browser).
I.e. Crypt of the NecroDancer: AMPLIFIED for Beta Testing Oh! "Beta Testing" in the name!
But this is a vaild key for SteamGifts, because ReleaseStateOverride = released <- that mean that developer marked all "beta" keys as release key and they will not be revoked.

In 99.99% of time it's all good.
Generally it's a key given away by producer for review and sometime in case of multiple key given for giveaways too.

If you received the good game, it should be fine ;)

They should be good. I bought a couple of Steam games from Humble Bundle during a sale some years ago, that once activated they said "for Beta Testing" (one of them was "Talewind", but I can't remember the other one...) and I had to contact support to ask about them if they were okay or something, and they said something like, "It's okay. If you have access to the full game all is okay. Those keys are meant to access the game for testing for a time, but once that period is over, it's normal for publishers to allow permanent access to the game with the remaining keys and redistribute them."

Humble Bundle also said that sometimes they receive those type of keys, and if I had any issues with the games, I should contact them again. I didn't, because I didn't have any issues with those. I remember downloading and playing the full versions of those games after that, and they have never been revoked, so far.