libsnapshot: Add unit test to simulate merge-resume

Simulate merge interruption and merge restart and
validate the data once entire merge is completed.

Bug: 167409187
Test: cow_snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ia940d5fbd2426bdf13347ffb6637d753b2228de6