mgr/dashboard: Refactor Python unittests

mgr/dashboard: Refactor Python unittests

* Make use of the KVStoreMockMixin class to get rid off duplicate code.
* Fake the index.html file to be able to run tests/test_home.py locally without building the frontend in production mode.
* Encapsulate helper functions in controllers/home.py, otherwise tests/test_feature_toggles.py need to fake the filesystem because load_controllers() will load the home.py controller and fail due missing files in the filesystem.

Signed-off-by: Volker Theile <vtheile@suse.com>