chore: add missing test fixture and changelog entry

We somehow forgot to include these files in a previous commit...

changelog.d/20240130_123351_regis_fix_save_on_plugins.md

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+- [Bugfix] Actually update the environment on `tutor plugins enable ...`. (by @regisb)
+- [Feature] Introduce a `tutor.hooks.lru_cache` decorator that is automatically cleared whenever a plugin is loaded or unloaded. This is useful, in particular when a plugin implements a costly function that depends on tutor hooks. (by @regisb)

tests/test_plugins.py

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+from __future__ import annotations
+
+from tests.helpers import PluginsTestCase
+from tutor import hooks, plugins
+
+
+class PluginsTests(PluginsTestCase):
+    def test_env_patches_updated_on_new_plugin(self) -> None:
+        self.assertEqual([], list(plugins.iter_patches("mypatch")))
+
+        hooks.Filters.ENV_PATCHES.add_item(("mypatch", "hello!"))
+
+        # env patches cache should be cleared on new plugin
+        hooks.Actions.PLUGIN_LOADED.do("dummyplugin")
+
+        self.assertEqual(["hello!"], list(plugins.iter_patches("mypatch")))