[testing] Add most testing via tox.ini

Consolidate testing, so that things can be run from CLI, CI and
various other means in a consistent way by using tox.

* Updated all the flake8 tests in the tests folder.
* Added pylint as a neutral test so that we can work on this in stages
  and have some collaboration on what we test and don't
* The tox tests for unit, stageone, stagetwo testing makes is easier
  for users to know how to run tests, and not have to do things
  manually
* Using tox for CI doesn't make sense, as that will create virtual
  envs and will disregard system/snap based python modules so may not
  work

Signed-off-by: Arif Ali <arif.ali@canonical.com>

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