snapuserd: Add test case

Add unit test to test reading dm-user block device
without user-space daemon. IO's should fail
with IO error.

Bug: 188362117
Test: cow_snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ida4efb4373904f6bb92f9904af9fe8443724bd2c