cros-test: Support companion DUTs

It is the second CL of a chain of CLs to enhance cros-test to support
multi-DUTs.
This CL add code to suport companion DUTs for both Tast & Tauto.
New unit test has been added.

BUG=b:199941891
TEST=./fast_build.sh -T; cros-test --input ...companion.json ....

Change-Id: Iab4188c9633b0b4f8e218b9851e1aee9757b21bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3216909
Commit-Queue: Seewai Fu <seewaifu@google.com>
Tested-by: Seewai Fu <seewaifu@google.com>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Jesse McGuire <jessemcguire@google.com>
8 files changed