Add unit tests

Problem/Motivation

All services and plugins provided by this module should have corresponding unit tests where applicable. Currently they are only tested via kernel tests.

Proposed resolution

• Add unit tests for services defined by this module (where applicable)
• Add unit tests for plugins defined by this module (where applicable)

Remaining tasks

• Create a patch

User interface changes

n/a

API changes

n/a

Data model changes

n/a

Release notes snippet

• Add corresponding unit tests for services provided by this module
• Add corresponding unit tests for plugins provided by this module
• Improved view mode options sorting by label (while ensuring 'default' always comes first)
• Improved view mode values sorting
• Various test cleanups and improvements