autotest: Modify netperf runner to tolerate failures

At high attenuations, we sometimes see netperf timeout.  This is
presumably because some control data needed to end the test is
having trouble getting from the server to the client or vice versa.
Fix this by optionally ignoring netperf command failures.  Callers
will have to adjust their code to tolerate None results from netperf
sessions.

TEST=RvR tests using netperf don't fail when netperf runs fail.  As a
pleasing side effect: since we might fail a TCP or UDP test before
running other TCP/UDP tests, this used to prevent us from running later
tests.  With this change, this doesn't happen.
BUG=chromium:282471

Change-Id: I927796cd5ff526900a0434479b343508d3587f9b
Reviewed-on: https://chromium-review.googlesource.com/167976
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
2 files changed