testsuite,ci: mark recv-discard-nullderef CI skip and tighten its check

testsuite,ci: mark recv-discard-nullderef CI skip and tighten its check

The regression test honestly skips when it cannot force the receiver's
output mkstemp() to fail -- as root (root bypasses DAC) and on Cygwin
(chmod 0555 does not deny the owner a write). The ubuntu, ubuntu-22.04,
almalinux and macOS jobs run `make check` as root, and Cygwin can't
enforce the unwritable directory, so the test skips on all of them.
runtests.py fails a run on any skip-set mismatch, so add the test to
those jobs' RSYNC_EXPECT_SKIPPED lists; the BSD/Solaris jobs run as root
too but enforce no expected-skip set, so they need no change.

Also tighten the pass condition. The post-chmod writability probe already
guarantees the receiver discards (mkstemp must fail), so an exit 0 would
mean the file actually transferred and the discard path was never
exercised -- a silent false-pass. Require exactly exit 23 (the forced
discard leaves the file untransferred); 12 remains the pre-fix crash.