Improve launching and reporting of tests and lint

[ceball]

Placeholder: fill in some content about a nice design that fits in with jupyter lab :) Are there other extensions we could be inspired by/steal from?

Currently:

• You launch a test run or lint check from the Commands box (which seems fine).
• You get a modal dialog asking if you want to run.
• After clicking OK, you get no feedback until the result appears.
• If the tests or lint don't complete within 5 seconds, you get an error (#211).
• If the tests or lint do complete within 5 seconds, you get an html report in a modal dialog.

[vidartf]

Some alternatives exist:

• Do everything in modal dialogs (not recommended).
• Create a main area widget that tracks progress and displays results.
• Create a side bar widget that tracks progress and displays results.

The choice between the last two should probably be informed by the questions:

• Should this always be readily available?
• Do you want to be able to run tests for multiple notebooks simultaneously, and/or keep the results of multiple notebooks and/or test runs in different tabs? etc.