Unit tests to make sure fonts are working as expected.

* Upgrade to the newest version of connect-fonts.
* Split off respondsWith into its own file so that it can be shared.
* For each locale, check whether all of the fonts are being served.
* For each served fonts.css, check to make sure embedded fonts are fetchable.
Merge pull request #3052 from shane-tomlinson/check-fonts-served

Unit tests to make sure fonts are working as expected.