test: add mfs interop tests

[achingbrain]

Requires mfs to be merged into js-ipfs first..

[achingbrain]

Needs the following merging first:

ipfs-inactive/js-ipfs-http-client#805
ipfs/js-ipfs#1360

[alanshaw]

LGTM, I'm getting some intermittent pubsub failures which are obviously unrelated

[daviddias]

Failing on Windows

[daviddias]

Also on Mac OS X with the same error.

  1) files does not error when creating the same directory twice and -p is passed:

     Error: file already exists

      at parseError (node_modules/ipfs-api/src/utils/send-request.js:17:17)

      at ClientRequest.<anonymous> (node_modules/ipfs-api/src/utils/send-request.js:47:14)

      at HTTPParser.parserOnIncomingClient (_http_client.js:546:21)

      at HTTPParser.parserOnHeadersComplete (_http_common.js:109:17)

      at Socket.socketOnData (_http_client.js:432:20)

      at addChunk (_stream_readable.js:283:12)

      at readableAddChunk (_stream_readable.js:264:11)

      at Socket.Readable.push (_stream_readable.js:219:10)

      at TCP.onread (net.js:635:20)

[achingbrain]

These are regressions caused by me trying to improve error message compatibility with go. It will need ipfs/js-ipfs#1439 merging & releasing first.

[achingbrain]

Those have been merged & released so this should be good to go, although now I think #23 is causing everything to catch fire ☹️

[alanshaw]

  1) files has the same hashes for small files with CIDv1:
      AssertionError: expected 'zb2rhiNedvrkpYhcrgtpmhKk5UPzcgizgSXaQLYXNY745BmYP' to deeply equal 'zdj7WcDSFNSsZkdkbpSDGeLsBtHbYKyvPQsaw6PpeeYdGqoAx'

All of the files tests are now passing for me when run with this PR to ipfs-unixfs-engine ipfs-inactive/js-ipfs-unixfs-engine#228