libsnaphot: Refactor cow_snapuserd test

Refact cow_snapuserd unit test. This will
test code changes in daemon in isolation. Currently it tests

1: Reading snapshot device and validate data
2: Merge and validate data.

More tests related to merge will be added later; but this
should provide a baseline unit test for daemon.

BUG: 168311203
Test: cow_snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I4d77c435a9f045204a4ff3ece524c857f9fba50f