libsnapshot_fuzzer: add new test directive to switch slot

Add a test directive to mimic the behavior of switching
slots after rebooting. This changes the behavior of
SnapshotFuzzDeviceInfo and has nothing to do with the
API surface of ISnapshotManager. It also relies on the
fact that SnapshotManager does not record the current
slot suffix and always reads from IDeviceInfo.

Test: run with corpus
Bug: 154633114
Change-Id: I6ebb68afb8519feaa05bd3e4817f0e06596fc920