Fix test-webkitpy on Python 3.6

https://bugs.webkit.org/show_bug.cgi?id=261115

Reviewed by Jonathan Bedard.

All of these failures are caused by the `sys.stderr.write` in
`webkitpy.__init__`. As such, when we run subprocesses in our tests, ignore
what's written to stderr.

* Tools/Scripts/webkitpy/common/system/executive_unittest.py:
(ExecutiveTest.test_run_command_with_unicode):
* Tools/Scripts/webkitpy/port/config_unittest.py:
(ConfigTest.test_default_configuration__standalone):

Canonical link: https://commits.webkit.org/267686@main

Tools/Scripts/webkitpy/common/system/executive_unittest.py

@@ -149,25 +149,26 @@ def test_run_command_with_unicode(self):
 
         executive = Executive()
 
-        output = executive.run_command(command_line('cat'), input=unicode_tor_input)
+        output = executive.run_command(command_line('cat'), input=unicode_tor_input, return_stderr=False)
         self.assertEqual(output, unicode_tor_output)
 
-        output = executive.run_command(command_line('echo', unicode_tor_input))
+        output = executive.run_command(command_line('echo', unicode_tor_input), return_stderr=False)
         self.assertEqual(output, unicode_tor_output)
 
-        output = executive.run_command(command_line('echo', unicode_tor_input), decode_output=False)
+        output = executive.run_command(command_line('echo', unicode_tor_input), decode_output=False, return_stderr=False)
         self.assertEqual(output, encoded_tor)
 
         # Make sure that str() input also works.
-        output = executive.run_command(command_line('cat'), input=encoded_tor, decode_output=False)
+        output = executive.run_command(command_line('cat'), input=encoded_tor, decode_output=False, return_stderr=False)
         self.assertEqual(output, encoded_tor)
 
         # FIXME: We should only have one run* method to test
+        # These use assertIn as run_and_throw_if_fail always redirects stderr to stdout
         output = executive.run_and_throw_if_fail(command_line('echo', unicode_tor_input), quiet=True)
-        self.assertEqual(output, unicode_tor_output)
+        self.assertIn(unicode_tor_output, output)
 
         output = executive.run_and_throw_if_fail(command_line('echo', unicode_tor_input), quiet=True, decode_output=False)
-        self.assertEqual(output, encoded_tor)
+        self.assertIn(encoded_tor, output)
 
     def serial_test_kill_process(self):
         executive = Executive()

Tools/Scripts/webkitpy/port/config_unittest.py

@@ -137,7 +137,7 @@ def test_default_configuration__standalone(self):
         expected = 'Debug'
 
         args = [sys.executable, script, '--mock', expected]
-        actual = e.run_command(args).rstrip()
+        actual = e.run_command(args, return_stderr=False).rstrip()
         self.assertEqual(actual, expected)
 
     def test_default_configuration__no_perl(self):