snapuserd: Add unit test for test merge code path

Most of the test cases are similar to dm-snapshot-merge.

Additional test cases have been added primarily to test
I/O's in parallel with merge.

Bug: 193863397
Test: snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I2764e6971989c121be873cc425cac464f31ce85f