Add test_libaom build target

This CL adds a target to build libaom unittests under the chromium
build system and with the partition allocator. This will allow the
test suite to take advantage of Arm's Memory Tagging Extension which
is already supported in the partition allocator.

Change-Id: I87805473c6b123918cd107ff9789325ceff7b0fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4498402
Reviewed-by: James Zern <jzern@google.com>
Reviewed-by: Wan-Teh Chang <wtc@google.com>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/main@{#1159614}