<!-- Thanks for submitting a pull request! Please provide enough
information so that others can review your pull request. -->
**What kind of change does this PR introduce?**
This is a change to a custom mocha runner to forward arguments to mocha.
**Summary**
In order to start using puppeteer custom mocha runner, the Firefox team
needs to forward some arguments to mocha through the runner to make it
compatible with our log parser.
**Does this PR introduce a breaking change?**
no
