Add base structure for testing of statuses from real cloud

[rgildein]

With these changes, we can simply add any result form real cloud to tests/unit/sample_plans directory in yaml file formatted like this:

plan: |
  Upgrade plan defined as string

applications:
  keystone:
    can_upgrade_to: ussuri/stable
    charm: keystone
    channel: ussuri/stable
    config:
      openstack-origin:
        value: distro
      action-managed-upgrade:
        value: true
    origin: ch
    series: focal
    subordinate_to: []
    workload_version: 17.0.1
    units:
      keystone/0:
        name: keystone/0
        machine: '0'
        workload_version: 17.0.1
        os_version: ussuri
    machines:
      '0':
        id: '0'
        apps: !!python/tuple ['keystone-ldap']
        az: az-0

My only two concerns are:
- usage of pytest-subtests, which result could be strange if test failed, as you can see in current CI After discussion I choose to got with only fixture returning dictionary with file as key, and tuple value with model and exp_plan. The model.get_applications returns applications defined in file.
- these test should be more part of functional tests instead of unit tests, or somehow exclude them from coverage report (e.g. 100% coverage is achieved without these tests) After discussions we agreed that new tox env should be created, so I created mocked-plans. I also add new jobs to run this environment to our GitHub workflow.

[rgildein]

@gabrielcocenza Thanks for doing #341. I merged, so I can use it here.

[agileshaw]

This is a partial review since I haven't finish going through all the changes yet.

[agileshaw]

Mostly looks good with some minor changes needed

[agileshaw]

LGTM, thanks

[aieri]

Thank you, I really like this work!

I have only left some non-blocking suggestions.

Additionally and as a sidenote, I don't like that we're tightly coupling what a plan is to its string representation; we should be able to change how a plan is represented without having to alter the tests that verify that it is correct. But of course this cannot be solved in this PR.

I want to merge this to ublock another tasks.