[TASK] Add accessor tests for OutputFormat (part 2)

[oliverklee]

Currently, this class uses magic getters and setters. These should be proper methods instead.

But first, we should have tests for the accessors. This is part 2.

[coveralls]

coverage: 45.347%. remained the same
when pulling 34495bb on feature/output-format-accessors-2
into dcb658c on main.

[JakeQZ]

One test method is missing Initially from its name.

Also wondering about ' ' as separator test values not being particularly real-world.

[JakeQZ]

An empty string or a newline might be a more realistic (non-default) real-world example.

But that said, maybe it's better to use multiple spaces in the tests, to avoid inadvertently using the default value - though newline is not the default value for any of these properties AFAIK.