[Lens] rewrite app.test.tsx tests to testing library

[mbondyra]

The tests in app.test.tsx file in lens plugin are the most complex, overengineered, overmocked and very difficult to understand. To add to that, they are also throwing a lot of warnings in the console. We should rewrite them to actually make them useful instead of painful.

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)