If we're here to help Riot find bugs, test features, etc, wouldn't that mean not playing a game normally?

I'd assume a tester would try all sorts of item/ability combinations on champions and many other mixups. This would be the equivalent of trolling in an actual match.

So where is the line between testing and trolling?

How is testing item combinations any form of testing? It basically achieves nothing outside of the viability of said combinations, and even then you can do that on customs, so this shouldn't be an issue anyhow.

The line is in custom games, where you can test all you want. generally you'll explore different features during normal games, and if you find something that conflicts, you'll get a custom started and try to reproduce the problem. If you can reproduce the issue semi-reliably, you post on the forums about the bug/glitch and ask people to confirm it with you, or by themselves in a custom.

I know this doesn't explain it too well, but the best way I can is to say don't buy something ineffective towards winning on other players time, it's almost guaranteed to royally disrupt someones cool.

~Summoner namora, of the lower case.