fix: validate client cert paths exist on disk before proceeding

Closes #486

See merge request gitlab-org/gitlab-shell!508

Logrus buffers its output internally, which makes these tests fail
intermittently. They're also not a good example to follow generally.

We now have acceptance tests that exercise this functionality so I'm
pretty relaxed about losing the expectations. However, we can test
them by inspecting the server-received metadata too, so there's no loss
of coverage here.

The move from logrus to labkit for logging also makes these tests hard
to justify keeping.