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.

Instead of completely giving up on the first failure, ask for
credentials as long as we fail to authenticate.

An empty string is not a valid number, and some shells complain.

Check instead if $COVERITY is non-empty, which is a common convention
and what we're doing anyway.

When implementing the ssh testing, the move to the script made it so
the first test suite's exit code was ignored. Check whether the main
tests fail and exit with an error in that case.