Improve Unit Test Coverage of ArrayReaderBuilder

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Whilst working on #1483 I noticed that ArrayReaderBuilder has a single rather limited unit test. Whilst I think many codepaths are covered by tests at higher levels, it would be good to have some unit test coverage of what is a relatively non-trivial piece of code which walks the potentially nested parquet schema.

It may also be a worthwhile exercise to split up the relatively large functions into smaller functions that can be tested independently, with the added bonus of improving readability.

Describe the solution you'd like

Some more unit tests of this functionality

[tustvold]

#1682 greatly simplified this logic, moving it to be covered by the pre-existing schema test suite, and removed ArrayReaderBuilder.

#1698 tracks some additional improvements that could be made to further enhance the test coverage of the schema conversion logic, but I think #1682 can be considered closing this ticket