Remove vendored test files

Historical `godep save` calls have included the `-t` argument, which saves test
files as well as code. Unfortunately, one of our dependencies (go-dockerclient)
has vendored code with rewritten paths for a testify that they didn't vendor.

This causes Godep to error on some `godep save -t` calls, as it can't find
`github.com/fsouza/go-dockerclient/external/github.com/stretchr/testify/assert`.

If we're never going to run these tests, there's no need to vendor them in
anyway.