[refactor] unit tests to utilize paramaterized and break down monolithic tests

- for tests which perform the same arrange/act/assert pattern but with different
  data, the data portion has been moved to the ``paramaterized.expand`` fields

- for monolithic tests which performed multiple arrange/act/asserts,
  they have been broken up into different unit tests.

- when possible, change generic assert statements to more concise
  asserts (i.e. ``assertIsNone``)

This work ultimately is focused on creating smaller and more concise tests.
While paramaterized may make adding new configurations for existing tests
easier, that is just a beneficial side effect.  The main benefit is that smaller
tests are easier to reason about, meaning they are easier to debug when they
start failing.  This improves the developer experience in debugging what went
wrong when refactoring the project.

Total number of tests went from 192 -> 259; or, broke apart larger tests into 69
more concise ones.

tests/unit/test_preferences.py

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # pylint: disable=missing-module-docstring, invalid-name
 
+from tests import SearxTestCase
 from searx.locales import locales_initialize
 from searx.preferences import (
     EnumStringSetting,
@@ -10,12 +11,12 @@ from searx.preferences import (
     PluginsSetting,
     ValidationException,
 )
-from tests import SearxTestCase
+from searx.plugins import Plugin
 
 locales_initialize()
 
 
-class PluginStub:  # pylint: disable=missing-class-docstring, too-few-public-methods
+class PluginStub(Plugin):  # pylint: disable=missing-class-docstring, too-few-public-methods
     def __init__(self, plugin_id, default_on):
         self.id = plugin_id
         self.default_on = default_on
@@ -47,22 +48,22 @@ class TestSettings(SearxTestCase):  # pylint: disable=missing-class-docstring
 
     def test_enum_setting_invalid_default_value(self):
         with self.assertRaises(ValidationException):
-            EnumStringSetting(3, choices=[0, 1, 2])
+            EnumStringSetting('3', choices=['0', '1', '2'])
 
     def test_enum_setting_invalid_choice(self):
-        setting = EnumStringSetting(0, choices=[0, 1, 2])
+        setting = EnumStringSetting('0', choices=['0', '1', '2'])
         with self.assertRaises(ValidationException):
-            setting.parse(3)
+            setting.parse('3')
 
     def test_enum_setting_valid_default(self):
-        setting = EnumStringSetting(3, choices=[1, 2, 3])
-        self.assertEqual(setting.get_value(), 3)
+        setting = EnumStringSetting('3', choices=['1', '2', '3'])
+        self.assertEqual(setting.get_value(), '3')
 
     def test_enum_setting_valid_choice(self):
-        setting = EnumStringSetting(3, choices=[1, 2, 3])
-        self.assertEqual(setting.get_value(), 3)
-        setting.parse(2)
-        self.assertEqual(setting.get_value(), 2)
+        setting = EnumStringSetting('3', choices=['1', '2', '3'])
+        self.assertEqual(setting.get_value(), '3')
+        setting.parse('2')
+        self.assertEqual(setting.get_value(), '2')
 
     # multiple choice settings