TaskCluster should be mocked for unittests.

[jschwartzentruber]

Currently we try fetching metadata for all supported builds from production TaskCluster.

We should mirror the requests made and commit them in so we can mock them with the built-in HTTP server implementation. We should also add stripped down data files (zip/tar.bz2/dmg) with 0-length contents to test extraction.

[jschwartzentruber]

Mock infrastructure added in 1927638.

Still need to write files & tests for extraction.