tests: Fix opt-starttls with nbdkit 1.30.10

Fedora 36 recently bumped to a version of nbdkit that fixed the bug
with NBD_OPT_INFO before NBD_OPT_STARTTLS, but without also
backporting the change to reject redundat NBD_OPT_STRUCTURED_REPLY.
Work around the difference by adding a sentinel value to the check()
function.