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
Reviewed-on: http://review.couchbase.org/41489
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>