Adding libfs_avb unittest framework

This commit adds the fundamental test framework for the upcoming
changes in libfs_avb. It replies on 'avbtool' to append AVB
metadata and then extracts the vbmeta content into an internal
|vbmeta_images_| variable for comparison later.

Bug: 117960205
Test: m libfs_avb_host_unittest
Test: ./out/host/linux-x86/nativetest/libfs_avb_host_unittest/libfs_avb_host_unittest
Test: atest --rebuild-module-info
Test: atest libfs_avb_host_unittest

Change-Id: I0cb8e8f04bc4b8628576565e7ff1920d7c81ecac