libsnapshot:snapuserd:Add unit test for read-ahead code path.

Add overlapping copy ops to test the read-ahead logic.

Bug: 183863613
Test: cow_snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ie96bc644c5f2eaae45cf048d9ba8a206930c3ce8