CMake: if pylint is available, run it as part of the test-suite

CMake: if pylint is available, run it as part of the test-suite

This introduces a stub-implementation (fake) that mimics the
API offered by libcalamares (the library is actually exposed
to Python via Boost::Python, so it doesn't act like a C-extension).
Using that stub-implementation, we can check Python modules for
validity as part of the test-suite.

The stub-implementation is needed, because otherwise every
Python module already fails at `import libcalamares`.

- stub-implement the API that is actually used by the Python modules
- in globalstorage, be slightly smart about what keys are being
  requested (so that e.g. all the modules that handle partitions
  information get an empty list and can manipulate that, instead of
  erroring out when they get a string)