More robust test script for use on CI environments

- Go & GOPATH are required
- gnatsd is downloaded & installed (if not already installed) into the
  existing GOPATH
- ginkgo is installed from Godeps into the Godeps workspace
- Godeps workspace is prepended onto PATH to prefer Godep dependency versions
  over local versions
- To use local dependencies, use `ginkgo -r` instead of `scripts/test`

[#63154330]