L2SS-259: Integrate unit tests using Continuous Integration (CI)

Tox is used to create virtual environments that can install Python dependencies and run tasks. These include linting and unit tests.

Tox uses stestr to run unit tests which runs all the tests concurrently.

This uses pbr for setup.py which automatically extracts features and entrypoints out of setup.cfg otherwise you would have to type this by hand. Together pbr and setup.cfg can also automatically tag the version of the package. For now I have left the lofar_git code in place, however.

For the documentation on Unit testing and support frameworks see: https://git.astron.nl/lofar2.0/tango/-/blob/L2SS-259-ci-unit-tests/devices/test/README.md

Closes L2SS-259

.gitlab-ci.yml

 image: artefact.skatelescope.org/ska-tango-images/tango-itango:9.3.3.7
 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
+  PBR_VERSION: "0.1"
 cache:
   paths:
     - .cache/pip