Replace some ASSERT_TRUE() asserts in DBTest.DynamicMemtableOptions and DynamicCompactionOptions with more specific ones

Summary: Replace some ASSERT_TRUE() to ASSERT_GT() and ASSERT_LT() so that in case the assert is triggered, the value is printed out.

Test Plan: Run the two tests

Reviewers: ljin, rven, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28293