QA: Remove `act` method

The conciseness offered by act is great, but the subtle-yet-important difference with perform also creates a small problem.

It's not difficult to swap act with perform or vice-versa when you get an error, but the QA test development cycle is already slow, so it can be frustrating losing a minute or two over a simple mistake that could be avoided completely by having only one way to do it.

Also it feels like an unexpected context switch when skimming through code that includes an act block.

I venture to guess that devs reading this who haven't written a GitLab QA test in a couple weeks are wondering "So, what exactly is the difference again"? Anyway, that's just how I feel when I come back to it.

Could/should we remove it to avoid these problems, and instead standardize on the more verbose perform?

cc @godfat