tests: Silence migration-test 'bad' test

In 2c9bb297 I added a migration test that purposely fails;
unfortunately it prints a copy of the failure message to stderr
which makes the output a bit messy.

Hide stderr for that test.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180306173042.24572-1-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>