Unit test wrong channel type

Related issue: #498 (closed)

!463 (merged) extracted connections into their own file, this merge request introduces a small test that verifies the rejection data when a passed channel has the wrong type.

Due to the concurrent nature of that part of the app: https://gitlab.com/gitlab-org/gitlab-shell/blob/d144db2951030a44ce787181ed8c67477dadb951/internal/sshd/connection.go#L69, this code calls the handle function asynchronously and introduces a wait group in order to synchronise the calls.