Add info on running tests and code style to Collaborator's Guide

[crandmck]

Per discussion in the TC, we need to update https://github.com/expressjs/express/blob/master/Collaborator-Guide.md with info on coding style (cf. expressjs/discussions#18) and how to run tests.

[thamaranth]

I can help with updating the docs if you'd like.

[thamaranth]

@dougwilson what do you think?

[dougwilson]

That'd be great! Let me know if you are looking for anything before getting started or need any pointers :)

[ryhinchey]

Looks like this issue can be closed