This test re-signs the virt apex contents and then check if they are
signed with the new key. Note that this isn't an end-to-end test to
install re-signed apex and see if it works, but only checks avbtool
signable items are signed with the new key by comparing pubkey digest.
This will help to notice when adding a new signed item without modifying
sign_virt_apex though.
Bug: 193504286
Test: atest --host sign_virt_apex_test
Change-Id: I16098c49f41e14f571dca272310b81459d9803a8
Jooyung Han