[th/steps-unify] merge nm{cli,tui}/features/steps/ directories

I understand that there seems to be a desire that we have separate "nmcli" and "nmtui" tests.

Why is that even?

I always found this leads to code duplication. I also think that we should have one set of steps, which make up a powerful utility library for testing. Having them duplicated under nm{cli,tui}/features/steps/ seems undesirable.

Already before I merged most of the steps implementations (and symlinked them together). Go further, and also symlink the rest that differed.

In the first moment this makes things uglier (if IS_NMTUI:), but in my opinion this give an opportunity to consolidate and cleanup code.

What do you think?

@os:rhel8.3