Fix grep_test:recurse with ZFS and TMPFS tmpdirs

contrib/netbsd-tests/usr.bin/grep/t_grep.sh
Fix grep_test:recurse when /tmp is either zfs or tmpfs. The test was
relying on an implicit ordering of directory recursion which happens
to be true when using UFS. grep's specification requires no such
ordering. The solution is to ignore the order of grep's results.

Reviewed by: ngie
MFC after: 32 days
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4925