[minor] make parquet prune tests more readable

[Ted-Jiang]

          It took me a while to convince myself that this was actually setting up the scenario as described. I eventually found it here:

https://github.com/alamb/arrow-datafusion/blob/dee926519030301f052dc2c3196e4fbef0da4c47/datafusion/core/tests/parquet/mod.rs#L916-L920

I wonder if it would be possible to add some better comments to test_prune mentioning that the created parquet files have 5 rows per page

Originally posted by @alamb in #10051 (comment)