All binary array tests generate fixed size binary arrays

[raunaks13]

Currently all tests for binary arrays end up generating binary arrays where all items have the same size (see RandomBinaryGenerator::generate() in generator.rs)

We should update this, (i) for the sake of exhaustive testing and (ii) because we should have separate tests for the fixed size binary encoding and the default binary encoding. Otherwise all tests that use this generator will use the fixed size encoding by default once it is enabled.