Add simpler unit tests for VerifiedMemory.

Also remember to free the reservation and the end of (non-negative) tests.

BUG=21469
TBR=whesse@google.com

Review URL: https://codereview.chromium.org//705713004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41528 260f80e4-7a28-3924-810f-c04153c831b5