test-webservice: Avoid leaking mock-sshd processes

With Linux PR_SET_PDEATHSIG, if the test program dies, that will
automatically terminate the child.  To make this work, we also need to
tell GLib not to double-fork, so it remains our child and not pid 1's.

Closes #163

Signed-off-by: Stef Walter <stefw@redhat.com>