mon: fix set cache_target_full_ratio

It was a noop because it was incorrectly using the variable n. Add a
test to protect against regression.

http://tracker.ceph.com/issues/8440 Fixes: #8440

Reported-by: Geoffrey Hartz <hartz.geoffrey@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>