pretend doesn't work with mirrors

[Melkor333]

Mirror creation/updating is still done when --pretend is used. It's because the mirror code currently doesn't use the Command feature but runs commands directly. I'm not sure if the solution is to unify the code and also use a Command for the mirroring (though there might be implications, not sure). Or if we should just propagate the config option to mirrors and check if pretend mode is activated.

@ganwell I don't know the pytest suite very well, but it already seems like we're generating tests. Is it possible to generate test cases that run with --pretend twice and make sure the output is exactly the same both times?

And since we're already at it, I'd propose renaming pretend to dryrun, it's much more popular.