Putting tests next to what will be tested

[SunkPlane29]

Currently, the tests written for this project are in a separate tests folder. It's best practice in Go putting each test inside the package that defines what is to be tested. For example:

service/
    youtube.go
    youtube_test.go
    extract_links.go
    extract_links_test.go

Note that there are some cases where tests can be put in a separate folder, but it's pretty uncommon.

We can use some open source projects for guidance:
https://github.com/kubernetes/kubernetes/tree/master/pkg/controlplane
https://github.com/tsuru/tsuru/tree/master/api
https://github.com/cockroachdb/cockroach/tree/master/pkg/base
https://github.com/golang/go/tree/master/src/bytes (Go's standard bytes package)