[DDW-425] Add env var to keep deadalus alive after tests

[DominikGuzei]

This PR adds the optional KEEP_APP_AFTER_TESTS=true environment flag which tells the test script not to close the app after all tests have run. This is useful to inspect the state of the app after all tests have been executed (and especially during development via test:watch)

Also updated the readme about testing with this new information + fixed older docs which are out of date now with the node ipc stuff.

Review Checklist:

Basics

• PR is updated to the most recent version of target branch (and there are no conflicts)
• PR has good description that summarizes all changes and shows some screenshots or animated GIFs of important UI changes
• CHANGELOG entry has been added and is linked to the correct PR on GitHub
• Automated tests: All acceptance tests are passing (yarn run test)
• Manual tests (minimum tests should cover newly added feature/fix): App works correctly in development build (yarn run dev)
• Manual tests (minimum tests should cover newly added feature/fix): App works correctly in production build (yarn run package / CI builds)
• There are no flow errors or warnings (yarn run flow:test)
• There are no lint errors or warnings (yarn run lint)
• Text changes are proofread and approved (Jane Wild)
• There are no missing translations (running yarn run manage:translations produces no changes)
• UI changes look good in all themes (Alexander Rukin)
• Storybook works and no stories are broken (yarn run storybook)
• In case of dependency changes yarn.lock file is updated

Code Quality

• Important parts of the code are properly documented and commented
• Code is properly typed with flow
• React components are split-up enough to avoid unnecessary re-rendering
• Any code that only works in Electron is neatly separated from components

Testing

• New feature / change is covered by acceptance tests
• All existing acceptance tests are still up-to-date
• New feature / change is covered by Daedalus Testing scenario
• All existing Daedalus Testing scenarios are still up-to-date

After Review:

• Merge PR
• Delete source branch
• Move ticket to done on the Youtrack board

[nikolaglumac]

Great stuff @DominikGuzei 👍

[thedanheller]

Great feature and docs improvement. 🙌