Expose CLI invocation reset

[LecrisUT]

I've found that in order to test tmt click command, we need to reset the cli invocation as: https://github.com/teemtee/tmt/blob/78742f93789da5bbc13db3713cd6de3e8f744443/tests/__init__.py#L8-L29

For external plugin support it would be good to expose this function in tmt.utils or have a generic tmt.testing to simplify writing tests. I can try to gather some more ideas for the latter as I stumble upon them if that is the preferred approach. So far I have:

• test fixture for cli_runner, maybe as tmt_cli
• test fixture to initialize or read a tmt tree. How to use this might be quite tricky since most of the api and testing goes through the cli wrappers, but maybe it can be useful in the future for preparing various other steps and only testing one step :thinking:
• lint helper
• caplog helper (maybe can already use the base one?)
• mock tmt.utils.Command.run. Probably that's the only mock helper needed?

[LecrisUT]

Actually a mock for Command.run would really be useful. I've been trying with flexmock, unittest.mock and pytest-subprocess. So far unsuccessful