Remove invalid UTF-8 characters from test output

Occasionally tests output characters that aren't valid unicode UTF-8
characters.  This causes the --json-test-results file not to be written.
Replace these characters with the UTF-8 invalid character.

BUG=

Review-Url: https://codereview.chromium.org/2086143003
Cr-Commit-Position: refs/heads/master@{#37404}

tools/testrunner/local/commands.py

@@ -111,8 +111,8 @@ def RunProcess(verbose, timeout, args, **rest):
   return output.Output(
       process.returncode,
       timeout_result[0],
-      stdout,
-      stderr,
+      stdout.decode('utf-8', 'replace').encode('utf-8'),
+      stderr.decode('utf-8', 'replace').encode('utf-8'),
       process.pid,
   )