ssh: Refactor SSHUploadPack tests to match modern best practices
In the context of https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15835 I was taking a look at our tests for SSHUploadPack and was taken aback a bit by the current state of these tests. I thus spent some time refactoring them so that they better match our current best practices and be hopefully a bit easier to extend going forward.