[autotest] Add an enum of priorities.

This is the definition of priorities that will be used across the rest
of the codebase.

BUG=chromium:250583
TEST=None

Change-Id: I718d221e5e533a4b60eac46231aa1c75490f055e
Reviewed-on: https://chromium-review.googlesource.com/168142
Reviewed-by: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
1 file changed