Add a test to perform block_image_update

Add the following tests:
stash src
bspatch stashed_src tgt
free stashed_src
(expected a successful update)

stash src
free stashed_src
fail_the_update
(expected stashed_src freed)

Bug: 36242722
Test: Test identified unfreed stashes correctly.
Change-Id: I5a136e8dc31774367972fbfe8c63cbc1ddb3a113