Add get_mapped_bytes() and release_free_memory() to testHarness

Allow clients of the testHarness to make use of these allocator
functions.

Change-Id: I8f3016caa4db466199d3caef25e466b1094a16a5
Add run_defragmenter_task to ENGINE API.

We need a way to trigger defragment on demand from testcases.

Change-Id: Ie11e0b4d2ad573361ed28416fdfda99f20d4fec2