libsnapshot_test: hardcode alignment.

SnapshotUpdateTest uses a relatively small super partition, which
requires a small alignment and 0 alignment offset to work. For the
purpose of this test, hardcode the alignment and offset.
This test isn't about testing liblp or libdm.

Fixes: 154355449
Fixes: 157437632
Bug: 158718136
Bug: 157633441
Bug: 154646936

Test: atest on devices with alignment of data partition >= 512KiB

Change-Id: I1d0474f028cc824bd4197d0228350395239b3b81
(cherry picked from commit 31739669e91c7a3afc5bee23e43bc45fb4154667)
Merged-In: I1d0474f028cc824bd4197d0228350395239b3b81