[Open Task] Add tests

[misteu]

GOAL

The app's code should be covered with tests.

WHY

Currently, the app is not tested at all.

ACCEPTANCE CRITERIA

Add tests for any parts of the app. Please start with Unit tests of critical parts of the app (e.g. saving, loading vocabulary). UI tests are not a priority right now.

1. Let me know (comment here) for which part(s) you want to add tests
2. I'll create an issue for you and assign you to that issue
3. Create a PR with your changes. If you had to make any changes to the actual code (e.g. to make it testable) please add this in the PR. Please do not just refactor parts of the code for the sake of it. We can safely do that as soon as tests are there :)

[yogesh-9999]

Hey @misteu assign this to me

[misteu]

Hey @yogesh-9999 Please focus on one issue after another. I assigned you already to https://github.com/misteu/VocabularyTraining/issues/20