autotest: Add a shortened netperf test tag property

This makes it easy to record concise performance keyval pairs.  We want
this for both the configuration and the result objects, so refactor
functions to generate these strings into static functions.  While we're
here, reorder properties into alphabetical order.

TEST=A pending test that consumes this logic executes correctly.
BUG=chromium:279395

Change-Id: I41034cb069047b010719ac6829c7869687b8f26f
Reviewed-on: https://chromium-review.googlesource.com/66972
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
2 files changed