Added npm task for getting unit test coverage

[bsclifton]

Test plan

1. git clone git@github.com:brave/browser-laptop.git
2. cd browser-laptop
3. npm run unittest-cov
4. you should see a very nice color coded dump of all files and their coverage (see picture below)

Description

Added new unittest-cov npm task which reports (via CLI) code coverage using istanbul.

Auditors: @alexwykoff, @bbondy

Fixes #6473

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Notes

• common options were moved to test/mocha.opts
• unit tests work great (with and without coverage)
• webdriver tests should not be impacted, but the npm task was edited. Let's watch tests on this one
• we'll want to understand more about coverage, but this should be a good start

Example output

[bsclifton]

Now that we can get coverage, we can look at services like these (which are free for open source projects!):

• Codacy (recommendation by @NejcZdovc; looks great!)
• Coveralls

[NejcZdovc]

One that is also very strong is Codecov

[bsclifton]

I re-ran failing tests; the ones which fail are also failing in master (they are not because of this). I did take a few minutes to make the alert tests more reliable.

Since this is very low impact and haven't gotten a review, I'll go ahead and merge