better test harness for reporters

[boneskull]

There's a lot of duplicated code in the reporter tests. Let's extract that into helper modules/functions/etc.

[jmuzsik]

Hi! I'd be willing to do this. I mean, do people get assigned in this repo or is it just a race to finish the issue. First come, first serve?

[Bamieh]

@jmuzsik Hello!, thanks for your interest to contribute 🎉. Just say you're going to work on it and start. Here is how it generally works in all OSS; You fork the project and create a new branch from master with a link to the issue number you are working as a commit message and push it before writing any code. This will link your branch to this issue, letting people know that you are working on it and follow on the progress.

example:

$ git commit -am "Closes https://github.com/mochajs/mocha/issues/3260"

If you need comments on the code or your branch is ready, submit a PR and we'll have a look at it to get it merged!

Cheers