Add tests for read_block_adb.

These tests aren't completely representative in that they don't run in
the recovery image. We might want to look in to adding a self-test
option to the recovery UI. Until then, these can be run on a normal
device (which is easier to do anyway).

Bug: 19522788
Change-Id: Idb20feb55d10c62905c2480ab1b61a2e4b5f60d8