libsnapshot: Add a simple script and gtest to simulate power failures during snapshot-merge.

This test is pretty straight forward: create two large files, then merge
one into the other while injecting kernel crashes. At the end,
block-by-block validation is performed against both the snapshot-merge target
and the file written to disk.

Bug: 139092356
Test: instructions in PowerTest.md
Change-Id: I28cc88160cf824039966abff4e61382090a354c5