t/cindex: fix broken size checks

t/cindex: fix broken size checks

Xapian size checks aren't accurate unless the shards are compacted.
Unfortunately, I commented out the -compact calls while working on
--split-shards but forgot to re-add them :x   So re-enable the
-compact calls and add an extra flag so we can skip size checks
when xapian-compact(1) is missing.

Fixes: 622e8c89 (*search: introduce open.lock for reader safety, 2025-10-08)