[testing/test_env.py] Update run_command_with_output to use rb and wb for processing log

This is based on Dirk's comment in https://chromium-review.googlesource.com/c/chromium/src/+/1259475#message-01cdaf3daae02840bb55b523a0eb71c9201adade

[email protected], [email protected]

Bug: 891747
Change-Id: Iec031e348d26d77de3f897641f4a151d0b4f544d
Reviewed-on: https://chromium-review.googlesource.com/c/1260519
Reviewed-by: Ned Nguyen <[email protected]>
Commit-Queue: Ned Nguyen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#596509}
1 file changed