Show invalid ini keys sorted

Otherwise this relies on the dictionary order of `config.inicfg`, which
is insertion order in py36+ but "random" order in py35.

src/_pytest/config/__init__.py

@@ -1074,7 +1074,7 @@ def _checkversion(self):
                 )
 
     def _validatekeys(self):
-        for key in self._get_unknown_ini_keys():
+        for key in sorted(self._get_unknown_ini_keys()):
             message = "Unknown config ini key: {}\n".format(key)
             if self.known_args_namespace.strict_config:
                 fail(message, pytrace=False)

testing/test_config.py

@@ -158,10 +158,10 @@ def test_confcutdir(self, testdir):
           """,
                 ["unknown_ini", "another_unknown_ini"],
                 [
-                    "WARNING: Unknown config ini key: unknown_ini",
                     "WARNING: Unknown config ini key: another_unknown_ini",
+                    "WARNING: Unknown config ini key: unknown_ini",
                 ],
-                "Unknown config ini key: unknown_ini",
+                "Unknown config ini key: another_unknown_ini",
             ),
             (
                 """
@@ -200,9 +200,7 @@ def test_invalid_ini_keys(
     ):
         testdir.tmpdir.join("pytest.ini").write(textwrap.dedent(ini_file_text))
         config = testdir.parseconfig()
-        assert config._get_unknown_ini_keys() == invalid_keys, str(
-            config._get_unknown_ini_keys()
-        )
+        assert sorted(config._get_unknown_ini_keys()) == sorted(invalid_keys)
 
         result = testdir.runpytest()
         result.stderr.fnmatch_lines(stderr_output)