Add App tests, adjusting App slightly.

Don't draw on screen during `App` tests. Test Sidekiq configuration
called. Test `App` hooks called, using `run_and_kill` helper.
Add Translatable tests.
Rework App #run_and_kill tests as #start_and_stop_app .

The `#run_and_kill` implementation failed intermittently. Rework as
`#start_and_stop_app`, explicitly stopping `App` and waiting a little
for it to end. Also fix typo in App tests description.
Add Display::Subpanel tests, tweaking Display::Subpanel.

Test main functionality of `Display::Subpanel`, including positioning
and data using strings and lambdas. Tweak `Display::Subpanel`, adding
`@data` empty-string defaults and simplifying slightly.