libavb: Add test for verifying veritymode is set to 'disabled'.

If the HASHTREE_DISABLED flag is set in the top-level vbmeta struct we
should set androidboot.veritymode=disabled no matter what is passed as
the AvbHashtreeErrorMode. Add unit test for checking this.

Bug: None
Test: New unit tests + all unit tests pass.
Test: Manually tested on UEFI-based boot loader.
Change-Id: I87c42b494562033954060fb270c064509beebc01