Use interface-ipfs-core tests for files API over http

[daviddias]

Uncomment https://github.com/ipfs/js-ipfs/blob/master/test/http-api/interface-ipfs-core-over-ipfs-api/test-files.js#L5-L24 and make it work.

Needs:

• .add/files.add http-api endpoint #323
• Upgrade the files branch (#323) to work with pull-streams #469

[victorb]

#323 closed in favor of #469 which includes the interface-ipfs-core changes

[daviddias]

woot! This is done, thank you @victorbjelkholm et all :D