Improve stage-test script: Split charms list according to tox envlist.

For newer charms the tox envlist includes lint, unit and integration: for these charms execute only lint and unit tests.
For older charms the tox envlist includes black, cover, flake8, pylint, yamllint, safety: for these charms execute all tests.

Change-Id: I6cfbe129440be1665f63572a1879060eccd822fd
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>