libsnapshot: Fix test failure in CancelOnTargetSlot.

This test fails because CreateLogicalPartitions is effectively called
twice without unmapping anything.

Bug: 208944665
Test: run_snapshot_tests.sh
Change-Id: I7e79f43e8de2f79bb72888c4198a95d9bd76bcdd
diff --git a/fs_mgr/libsnapshot/snapshot_test.cpp b/fs_mgr/libsnapshot/snapshot_test.cpp
index f1d76e7..c3b2f78 100644
--- a/fs_mgr/libsnapshot/snapshot_test.cpp
+++ b/fs_mgr/libsnapshot/snapshot_test.cpp
@@ -2215,6 +2215,8 @@
 TEST_F(SnapshotUpdateTest, CancelOnTargetSlot) {
     AddOperationForPartitions();
 
+    ASSERT_TRUE(UnmapAll());
+
     // Execute the update from B->A.
     test_device->set_slot_suffix("_b");
     ASSERT_TRUE(sm->BeginUpdate());