Improve `showExamsPage` test suite

After the newest updates of showExamsPage handler (https://github.com/learningequality/kolibri/pull/11570), its test suite started failing due to problems with set up. I needed to dig deep into various nested store actions to understand and locate several places that needed to be mocked. After seeing that there are even more places to be mocked after a significant period of time I spent on it, I gave up and disabled tests temporarily here https://github.com/learningequality/kolibri/pull/11570/commits/ff7a7baf34208edf97d192f6a2b5bd9f28bcd9e1. In my humble opinion, we need to re-design tests to

• Stop relying on internal Vuex store structure
• Be designed clearly so future updates are straightforward and not time-consuming
• Overall have more clarity about what's the purpose and dependencies of those tests

As we're moving towards composition API, this could be done as part of that work. The logic of this handler could be refactored to stop relying on Vuex for storing data whenever appropriate. When testing, we'd only feed the composable with some input and tested its output.

Start with the showExamsPage handler and the tests disabled in PR #11570 at commit ff7a7baf34208edf97d192f6a2b5bd9f28bcd9e1. Trace the nested Vuex store actions and the handler's dependencies before deciding how composition API work could isolate inputs and outputs. Done means the tests are re-enabled, avoid relying on internal Vuex structure, and clearly document their purpose and dependencies.