Fix run_dev_server_tests logging.
Fix run_dev_server_tests logging.

Before:
ERROR:root:Server stderr:
ERROR:root:SKIPPED  tracing.base.unittest.test_case_test.skipped_is_skipped
FLAKY  tracing.ui.tracks.drawing_container_perf_test.draw_softwareCanvas_One
FLAKY  tracing.ui.tracks.drawing_container_perf_test.draw_softwareCanvas_Ten

ERROR:root:Server stdout:
ERROR:root:PASSED  tracing.base.base64_test.getDecodedLength
PASSED  tracing.base.base64_test.DecodeToTypedArray

After:
ERROR:root:Server stderr:
EXCEPT  tracing.base.base64_test.getDecodedLength AssertionError: assert.fail()
... traceback redacted
FAILED  tracing.base.base64_test.getDecodedLength
FLAKY  tracing.ui.tracks.drawing_container_perf_test.draw_softwareCanvas_One
FLAKY  tracing.ui.tracks.drawing_container_perf_test.draw_softwareCanvas_Ten

ERROR:root:Server stdout:
PASSED  tracing.base.base64_test.DecodeToTypedArray

BUG=catapult:#3465

Review-Url: https://codereview.chromium.org/2783373003

catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,sahiljain/catapult,sahiljain/catapult,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult