Refactor tests

[stoically]

Here's a test change only - updated with the latest dependencies - which was previously in #1190, but since it doesn't touch production code, I think it's good to merge into master.

[maxxcrawford]

@stoically I'm not able to run this patch with all passing tests. Is there something wrong on my end or is that a known issue with this patch?

[stoically]

@maxxcrawford You're right, it seems something introduced a regression since I've opened the PR (travis was green back then). Reopening to trigger a red build and looking into it when I get the chance.

[stoically]

Interesting. So travis is green again, but locally creating a new container should create it in the browser as well fails for me. Odd.

[stoically]

@maxxcrawford rebased and updated to latest webextensions-jsdom, which depends on latest webextensions-api-fake, and now works for me locally as expected. Could you give it another look? Thanks!

[maxxcrawford]

Thanks for working on these test rewrites. I'm trying to get a better sense of this testing suite (and the project as a whole). Could you break this PR into multiple commits for better readability? Specifically, separating the coverage, dependencies updates and each test (where applicable). Still one PR, but enough different commits to step through the refactor easier.

The good news is that Mozilla has bandwidth (and an intern!) to put towards this project over the next three months.

[stoically]

Separated the changes into multiple commits as requested, hope this helps with readability. As an aside: Facebook Container uses the dependency webextensions-jsdom as well.

Looking forward to see progress in MAC!

[maxxcrawford]

Thanks for all the work on this refactor!