TestPluginFunctions: ensure we call parent setUp() before using `conf…

…ig.config`

```
setting = {'enabled_plugins': []}, persist = None, profiles = None

    @staticmethod
    def set_config_values(setting=None, persist=None, profiles=None):
        if setting:
            for key, value in setting.items():
>               config.config.setting[key] = value
E               AttributeError: 'NoneType' object has no attribute 'setting'

test/picardtestcase.py:110: AttributeError
```

test/test_plugins.py

@@ -295,6 +295,7 @@ def test_is_user_installed(self):
 
 class TestPluginFunctions(PicardTestCase):
     def setUp(self):
+        super().setUp()
         self.set_config_values({
             'enabled_plugins': [],
         })