[writeboost] Add test_invalidate_prev_cache

Writeboost usually writes back cache blocks in background but
there is a slow path (marked as unlikely) in foregruond which
performs single writeback.

To see how badly this path performs, this test submits 512B writes
in stride access patten and thus always pass through the inefficient path.

An experiment shows that throughtput decreses from 387KB/sec (HDD only) to
293KB/sec (With Writeboost) which is 25% loss.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>