libsnapshot: Fix MapAllSnapshotsWithoutSlotSwitch test

IsUserspaceSnapshotUpdateInProgress() would return dm partitions
which are backed by verity; this check is invalid for tests.

Bug: 369964380
Test: vts_libsnapshot_test64 --gtest_filter=SnapshotUpdateTest.MapAllSnapshotsWithoutSlotSwitch
Change-Id: I7d207c472077042ea07b65bf81b64c9c240ae8c7
Signed-off-by: Akilesh Kailash <akailash@google.com>
(cherry picked from commit 68c2cbf2)