[testing/test_env.py] Use utf-8 encoding for reading test log

Bug: 891747
Change-Id: I73142c8bb5c4bbc78da71f230188e33d548a3dab
Reviewed-on: https://chromium-review.googlesource.com/c/1259475
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596397}

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') as writer, io.open(stdoutfile, 'r', 1) as \
-      reader:
+  with io.open(stdoutfile, 'w', encoding='utf-8') as writer, \
+    io.open(stdoutfile, 'r', 1, encoding='utf-8') as reader:
     process = subprocess.Popen(argv, env=env, cwd=cwd, stdout=writer,
         stderr=subprocess.STDOUT)
     forward_signals([process])