Summary:
After recent change of DBTest.DynamicCompactionOptions, occasionally hit another non-deterministic case where L0 showdown is triggered while timeout should not triggered for hard limit.
Fix it by increasing L0 slowdown trigger at the same time.

Test Plan: Run the failed test.

Reviewers: igor, rven

Reviewed By: rven

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D36219