Add tests for launcher/UpdaterApp.py

[emkll]

Related to #646

We currently, by policy, targeting 100% test coverage for Updater.py, which contains most of updater logic.

While UpdaterApp.py primarily handles the UI, we should still consider adding test coverage there to prevent regressions, given that we are introducing new functionality and further complexity (e.g. handling migrations)

[zenmonkeykstop]

Test coverage has been increased from 2022 on. Closing, but test coverage for new launcher should also be at acceptable levels.