[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

TBR=dpranke@chromium.org, jbudorick@chromium.org

Bug: 891747
Change-Id: Iec031e348d26d77de3f897641f4a151d0b4f544d
Reviewed-on: https://chromium-review.googlesource.com/c/1260519Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#596509}

testing/test_env.py

@@ -178,8 +178,8 @@ def run_command_with_output(argv, stdoutfile, env=None, cwd=None):
   """
   print('Running %r in %r (env: %r)' % (argv, cwd, env))
   assert stdoutfile
-  with io.open(stdoutfile, 'w', encoding='utf-8') as writer, \
-    io.open(stdoutfile, 'r', 1, encoding='utf-8') as reader:
+  with io.open(stdoutfile, 'wb') as writer, \
+      io.open(stdoutfile, 'rb', 1) as reader:
     process = subprocess.Popen(argv, env=env, cwd=cwd, stdout=writer,
         stderr=subprocess.STDOUT)
     forward_signals([process])