When a user adds a registry (e.g., through a command like `ks registry
add`, or implicitly through `ks init`) we must parse a URI pointing at
registry hosted on github.com, since we currently only support the
`"github"` protocol.

This logic is somewhat complex, since we must

  1. infer the location of a `registry.yaml` file given the URI, and
  2. accept and parse a broad set of URIs a user might provide.

For example, consider the following valid URIs, and what we do to infer
the location of the `registry.yaml` file that specifies a registry:

  URIs with an explicit `registry.yaml`:
    github.com/exampleOrg/exampleRepo/blob/master/registry.yaml
    github.com/exampleOrg/exampleRepo/blob/master/incubator/registry.yaml

  URIs with an implicit `registry.yaml`:
    github.com/exampleOrg/exampleRepo/
    github.com/exampleOrg/exampleRepo/tree/master

  URIs with different protocols (or no protocol specified)
    github.com/exampleOrg/exampleRepo
    http://github.com/exampleOrg/exampleRepo
    https://github.com/exampleOrg/exampleRepo


    www.github.com/exampleOrg/exampleRepo

And so on.

Because this parsing logic has many branches, it is important for us to
test every branch, so that the user is not surprised when a command like
`registry add` doesn't work.

This commit will introduce such tests.

Signed-off-by: Alex Clemmer <clemmer.alexander@gmail.com>