subprocess2: Rewrite tests to be Python 3 compatible.

Also replace mox with mock and move the test script to a different file.

Bug: 984182
Change-Id: I9005a523c2abd82c38a7c61501c7cbfd4201a5b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1745412Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

testing_support/subprocess2_test_script.py

+#!/usr/bin/env python
+# Copyright (c) 2019 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Script used to test subprocess2."""
+
+import optparse
+import os
+import sys
+import time
+
+
+if sys.platform == 'win32':
+  # Annoying, make sure the output is not translated on Windows.
+  # pylint: disable=no-member,import-error
+  import msvcrt
+  msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
+  msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
+
+parser = optparse.OptionParser()
+parser.add_option(
+    '--fail',
+    dest='return_value',
+    action='store_const',
+    default=0,
+    const=64)
+parser.add_option(
+    '--crlf', action='store_const', const='\r\n', dest='eol', default='\n')
+parser.add_option(
+    '--cr', action='store_const', const='\r', dest='eol')
+parser.add_option('--stdout', action='store_true')
+parser.add_option('--stderr', action='store_true')
+parser.add_option('--read', action='store_true')
+options, args = parser.parse_args()
+if args:
+  parser.error('Internal error')
+
+def do(string):
+  if options.stdout:
+    sys.stdout.write(string.upper())
+    sys.stdout.write(options.eol)
+  if options.stderr:
+    sys.stderr.write(string.lower())
+    sys.stderr.write(options.eol)
+
+do('A')
+do('BB')
+do('CCC')
+if options.read:
+  assert options.return_value is 0
+  try:
+    while sys.stdin.read(1):
+      options.return_value += 1
+  except OSError:
+    pass
+
+sys.exit(options.return_value)