[Variant] `variant_get` tests clean up

[sdf-jkl]

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

Reading changes in #9498 I noticed that the macros mention not using variant_shred because it's not implemented yet.

Describe the solution you'd like

• Use variant_shred in these macros to make the code easier to read.
• Add some VariantArray::from_parts() while we're at it.

Describe alternatives you've considered

Additional context

[alamb]

label_issue.py automatically added labels {'parquet-variant'} from #9518