deprecate `tach test` in favor of pytest plugin

[DetachHead]

currenly tach test is implemented as a pytest plugin wrapped by a separate CLI, but this pytest plugin does not seem to be documented, so it seems to be an implementation detail and not intended to be used directly (since it also doesn't register itself using project.entry-points.pytest11 like other plugins do).

i think tach test would be better off as a pytest plugin because it makes it easier to use with IDEs that already have pytest integration, as well as integrate with other pytest plugins

[andre-vauvelle]

I agree, since it was broken previous to #885. I doubt anyone is using it currently.