Forward npm test arguments to mocha

[cirosantilli]

This commit allows running for example just:

npm test -- -g 'authentication'

instead of the previously needed:

npm test -- -- -- -g 'authentication'

since each npm run consumes one --.

Related: https://stackoverflow.com/questions/40495116/how-to-pass-a-command-line-argument-to-a-nested-script

Maybe this should be done on the generator instead instead of in this repo, not sure.