tests: refactor MockUrlFetcher to use files

[nokome]

The MockUrlFetcher maps URLs to an Object representing the expected JSON response.

Currently, adding a new URL requires adding a new method/if statement to the code. Instead, we could have a folder called tests/mockUrls with files like http-crandb.r-pkg.org-gsl.json with the expected JSON content. MockUrlFetcher would then just look for a file matching the URL and return a 404 if no match was found. That would make it easier to add/update the expected JSON as we add new fixtured e.g.

curl http://crandb.r-pkg.org/gsl > tests/mockUrls/http-crandb.r-pkg.org-gsl.json

[beneboy]

This is a good idea. I did this for the NPM information but in the interest of getting the tests fixed quickly I reverted to the if-statement approach.

[nokome]

Thanks @beneboy : would like to get the coverage up #46 before we go much further - this will make that easier.