Run shell scripts through shellcheck

The sshd on OSX has stopped responding to the setting which this test
assumes is there, so let's disable it until we can figure out what's
happening.

Test that the certificate check callback gets the right fingerprint from
the host we're connecting to.

This was added to avoid the remote's default branch to be considered to
the be notes one which the first network test leaves behind.

git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.

We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.

Our ssh tests assume that the server supports password authentication
in a few places. This is convenient as we're not testing authentication
methods, but what happens around them.

Tell sshd on OSX to accept this form of authentication.