Support for parallel test runners and output files

[fotinakis]

ember-exam can parallelize an Ember test suite by using multiple Chrome instances. However, because ember-cli-blanket uses the /write-coverage-report endpoint at the end of a test suite with a single output filename, only the last write will win.

If you could do something like specify a function that returns the output filename, and pass it an index as an argument, then you could write coverage-1.lcov, coverage-2.lcov, etc. This would work automatically with services like codecov.io that automatically merge report from parallel runners.

[sglanzer-deprecated]

I'm more than happy to accept a PR on this, but most of my future effort is going to be focused on ember-cli-code-coverage - if you would like to help contribute a parallelize feature to that project I'd be happy to work with you on it