disk: add detect_image_type unit tests
disk: add detect_image_type unit tests

BUG=None
TEST=cargo test -p disk --features=composite-disk

Change-Id: Ib1dd4cbf6b7f22dfa4ef5a8dcda3ca8679487884
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2824809
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm