The migration precopy testing helper function always expects the
migration to run to a completion state. There will be test scenarios
for TLS where expect either the client or server to fail the migration.
This expands the helper to cope with these scenarios.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220310171821.3724080-12-berrange@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>