test/libcephfs: use more entries to reproduce snapdiff fragmentation

test/libcephfs: use more entries to reproduce snapdiff fragmentation
issue.

Snapdiff listing fragments have different boundaries in Reef and Squid+
releases hence original reproducer (made for Reef) doesn't work properly
in S+ releases. This patch fixes that at cost of longer execution.
This might be redundant/senseless when backporting to Reef.

Related-to: https://tracker.ceph.com/issues/72518
Signed-off-by: Igor Fedotov <igor.fedotov@croit.io>