Add test-coverage

We are lacking test coverage, let's make an outline of what needs test coverage so we can split this up in smaller tasks.
Here's a start:

• #2736075: Add test coverage for a calendar view based on an entity property
• #2736077: Add test coverage for a calendar view based on a date field
• calendar_header area plugin
• datetime_year_week argument handler
• date_year_week argument hander
• calendar argument validator
• calendar pager
• calendar_row plugin
• calendar style plugin

It's really a rough overview. I think it would be best to start with the first 2 that should already cover most view plugins.