Label printing unit test (#3047)

* Adds a very simple sample plugin for label printing

* Test mixin install status and API query

* Better error reporting for label printing API

* pep fixes

* fix assertation

* remove broken assertation

* igonre for coverage

* test the base process of printing

* refactor tests

* clean up basic test

* refactor url

* fix url creation

* test printing multiples

* test all printing endpoints

* test all list options - move api tests

* test for invalid filters

* refactor

* test with no part

* these should not happen
checks are in place upstream

* fix assertation

* do not cover continue parts

* test for wrong implementation

* ignore DB not ready

* remove covage from default parts

* fix url generation

* test debug mode

* fix url assertation

* check that nothing was rendered

Co-authored-by: Oliver Walters <oliver.henry.walters@gmail.com>

InvenTree/plugin/samples/integration/custom_panel_sample.py

@@ -120,7 +120,7 @@ class CustomPanelSample(PanelMixin, SettingsMixin, InvenTreePlugin):
                         'icon': 'fa-user',
                         'content_template': 'panel_demo/childless.html',  # Note that the panel content is rendered using a template file!
                     })
-            except:
+            except:  # pragma: no cover
                 pass
 
         return panels