Force builtin plugins to be active (#4276)

* Force builtin plugins to be active

- Not setting the "active" flag can cause ambiguous errors
- e.g. some settings don't work if the plugin is inactive
- By design, builtin plugins are always "active" anyway
- This PR just forces the active field to True

* Fix unit tests

InvenTree/plugin/registry.py

@@ -408,6 +408,11 @@ class PluginsRegistry:
             # Check if this is a 'builtin' plugin
             builtin = plg.check_is_builtin()
 
+            # Auto-enable builtin plugins
+            if builtin and plg_db and not plg_db.active:
+                plg_db.active = True
+                plg_db.save()
+
             # Determine if this plugin should be loaded:
             # - If PLUGIN_TESTING is enabled
             # - If this is a 'builtin' plugin