tests(smoke): add smoke test code coverage

[brendankenny]

Since:

• tests(smokehouse): run smoketests in parallel #4748 moved the smoke test runner to pure js
• nyc supports merging code coverage from any tests run in child processes
• codecov supports uploading multiple coverage reports

it's now easy to add smoke test coverage :)

Run locally with yarn coverage:smoke and on codcov you get little unit/smoke tabs at the top of file views (e.g. report-generator.js). In codecov you seem to only get those tabs in individual files, both tabs are selected at first by default, and the coverage numbers on the left may be the sum of the two? So the view is a little shaky if you're interested in comparing unit vs smoke coverage, but it's nice that if you want the union of the two you can see that as well without uploading yet another coverage file.

[brendankenny]

oh, the big gotcha here is that if a function will be toString()ed and then passed to driver.evaluateAsync() or driver.evaluteScriptOnNewDocument() it must have a /* istanbul ignore next */ above it or nyc will instrument it and it will throw when then stringified and run in the browser.

Not an issue for (almost all of) the unit tests, but the smokehouse expectations will fail with gatherer errors if the code can't run. I can't think of a great way to automate doing this or checking it, so we'll rely on vigiliance?

[patrickhulce]

sweeeeeeeeet

[paulirish]

the big gotcha here is that if a function will be toString()ed

moving all these to *-pagefunctions.js will make this much more straightforward.