tests: Skip rather than fail on missing nbdkit --no-sr

CI was reporting various test failures on a debian-10 host, due to an
nbdkit older than 1.14 that introduced the --no-sr kill switch.  Skip
(portions of) tests in these cases.  For the language ports of test
250 (opt-set-meta), the second process isn't adding enough coverage to
be worth the hassle of porting requires() to non-C code, so drop it
instead.

Fixes: db3e8ada ("info: Add --is tls, --can structured-reply")
Fixes: cc60a2fa ("info: Expose whether structured replies were negotiated")
Fixes: 760c37c2 ("tests: Language port of nbd_opt_set_meta_context() tests")
Fixes: 97e6ab81 ("api: Add nbd_[aio_]opt_set_meta_context[_queries]")