tests: avoid slow ulimit -v behavior

tests: avoid slow ulimit -v behavior

* init.cfg (ulimit_supported_): skip_ if the ulimit -v
takes too long, which was seen with bash 5.2 on Solaris 11,
where fork() returned EAGAIN under memory constraints,
and bash retried for about 16 seconds.
(get_min_ulimit_v_): Break early if skipped.
* tests/misc/write-errors.sh: Be more conservative and
skip on failure to determine min ulimit.