Add an ArtifactType for fingerprint MCU unittests

The fingerprint MCU has some unittest binaries that we want to
run on-device. Add an ArtifactType for that so that CQ builders
can upload those binaries (and fingerprint Tast tests will download
them).

Due to https://crbug.com/1034529, add the new value in the Legacy
group. This should be moved to the Test group afterwards.

BUG=b:158580909
TEST=generate

Change-Id: Ieb1cd9ee3dd62a25e062b583ba6587b963e25f63
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2422169
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: Yicheng Li <yichengli@chromium.org>
2 files changed