Update hardcoded digests in avb_slot_verify_unittest to account for no salt

These values should have been updated in
https://android-review.googlesource.com/802635, but I must have run the
unit tests incorrectly (not sure how...) and missed the failures.

Bug: 120163069
Test: 'cd external/avb && atest libavb_host_unittest', verified 4 test
failures before change and all pass after

Change-Id: I29c54336344cd102e7742d8245258006b0a61fed