after changing pythonjs to mocha-headless-chrome grep on unit tests stopped working

[piotr-gawron]

After changing pythonjs to mocha-headless-chrome there was no way to run single test from the console.

For instance this call executed all unit tests, but shouldn't.

npm run test -- --grep=pileuputils

This PR restores grep functionality when running unit tests.

[coveralls]

Coverage remained the same at 91.456% when pulling f2d8fc2 on piotr-gawron:mocha-headless-chrome-grep-issue into 406edcb on hammerlab:master.

[akmorrow13]

Thanks for the catch, @piotr-gawron! I tested it out and it works great. Thanks for the contribution!