The testcase for this bug fails randomly due to two reasons.
1. Due to ibuf merge happening background
2. Due to dict stats update which brings the evicted page back into
buffer pool.

Fix ibuf_merge() to not do any merges with ibuf_debug enabled and
also changed dict_stats_update() to return fake statistics without brining
the secondary index pages into buffer pool.

Approved by Marko. rb#3420