cros_test_lib.OutputCapturer: use mock to patch stdout/stderr
cros_test_lib.OutputCapturer: use mock to patch stdout/stderr

Rather than monkey around with the stdout/stderr members ourselves,
use the mock.patch helper to do it for us.

BUG=chromium:344059
TEST=`./buidlbot/run_tests` passes

Change-Id: I3f7953f29373438448f3f1080ea00222c4f9814e
Reviewed-on: https://chromium-review.googlesource.com/186890
Reviewed-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite