Fix non-determinism in subprocess42_test.py.
Fix non-determinism in subprocess42_test.py.

Increase test coverage by explicitly testing recv_err() while at it.

A good source of non-determinism was the clock resolution on Windows, which may
be low enough that the clock wouldn't change when checking for the timeout.

Removed dead code.

Confirmed manually to pass on Windows 10, OSX 10.11 and Ubuntu 14.04.

R=kjlubick@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2291133004

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py