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. Additionally, MRs typically come in without -t.

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.