autotest: Emulate downstream UDP throughput in netperf config

netperf doesn't natively support a test type of UDP_MAERTS (stream
backwards) which means it lacks a button to let us measure UDP
downstream bandwidth.  We emulate this support by running the normal
STREAM test backwards (run the server on the DUT).  Make this interface
a little cleaner.

TEST=Tests using this logic behave as expected.
BUG=chromium:271515

Change-Id: Idce523d101d5725e786590be4c05082ae0bba9be
Reviewed-on: https://gerrit.chromium.org/gerrit/65710
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
2 files changed