[BUG] Plugin Schedule do not update when beeing changed + plugin testing (#4243)

* Add test for api_call

* Add coverage for LocateMixin

* no cov for no url

* make sure changed details get updated

* restructure code

* Test that changes in schedules are reflected
Fixes #4239

InvenTree/plugin/builtin/barcodes/inventree_barcode.py

@@ -55,7 +55,7 @@ class InvenTreeInternalBarcodePlugin(BarcodeMixin, InvenTreePlugin):
             url = instance.get_absolute_url()
             data['web_url'] = url
         else:
-            url = None
+            url = None  # pragma: no cover
 
         response = {
             label: data