Add filter module base, prepare adding filters (#553)

* Improve string handling.

* Cleanup tests.

* Add filter module mock.

plugins/plugin_utils/filter_module.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2022 Felix Fontein <felix@fontein.de>
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+# NOTE: THIS IS ONLY FOR FILTER PLUGINS!
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+
+from ansible.errors import AnsibleFilterError
+
+
+class FilterModuleMock(object):
+    def __init__(self, params):
+        self.check_mode = True
+        self.params = params
+        self._diff = False
+
+    def fail_json(self, msg, **kwargs):
+        raise AnsibleFilterError(msg)