msgmap: tmp_clone: use in-memory journal

msgmap: tmp_clone: use in-memory journal

This prevents $TMPDIR from being littered with *-journal files
after running the test suite.

This shouldn't cause excessive memory use since $v2w->{mm_tmp}
doesn't see big transactions.  There's no need to worry about
data loss, here,either, since this is just a temporary clone
we've even disabled fsync on.

Fixes: 78888d36fb80889f ("msgmap: use TRUNCATE for journal_mode, for now")