libsnapshot: Add vts_userspace_snapshot_test

Toggle virtual_ab.userspace.snapshots.enabled to test
both:

1: vts_libsnapshot_test - testing kernel dm-snapshot
2: vts_userspace_snapshot_test - testing user-space snapshot

Bug: 193863443
Test: vts_userspace_snapshot_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I873e7476d71feb9a2e12054e968443bd22ee817c