Make the tests workflow conigurable so we can run it against specific rocket.chat images

[jadolg]

It'd be of great use to the community if we could run the tests with a specific image so we can detect regressions and API problems in different versions.
Maybe test against all maintained versions of rocket.chat on PRs too

[jadolg]

Not exactly what's needed here but we started testing several versions now #261

This is a bit difficult to maintain since it'd need to be updated at least monthly. Let's see how well we can keep it up.