Add test for the device tree overlay mechanism.
Bug: b/296796644
Test: cd cros_fdt && cargo test
Change-Id: I8e8d129344e1ad4ddf0d7af7b3f21c0c9d9ca038
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4855966
Commit-Queue: Jakob Vukalović <jakobvukalovic@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>