Replace SCM mock tests about quickfeed HOT 3 OPEN

I looked into this a bit more, and this isn't unique to our case; others also want to unit/mock test the go-github library. We should check out if we can use the go-github-mock library.

from quickfeed.

That said, we are already using the github.com/steinfletcher/apitest library for some other github/web/auth testing.

from quickfeed.

Turns out the original MockSCM implementation is used by the web package to test things as well; so it is useful for that. But we should rewrite it so that requests and responses go through the GithubSCM implementation, instead of the MockSCM.

Turns out that we don't really need the go-github-mock library; we only need a few minor things from it and we can use the stdlib http.ServeMux and PathValue instead.

from quickfeed.