Consider removing RowConverterTest

RowConverter was previously untested. A test was added in https://github.com/gchq/sleeper/pull/7099. Consider whether this test is needed. Having it improves unit test coverage. The counter-argument is that "It couples very tightly to Parquet primitives, and doesn't define the end behaviour from Sleeper's perspective. "

Start by reviewing RowConverterTest and the test added in PR 7099, then assess whether it tests Sleeper-visible behavior or only Parquet primitives. Done means reaching and recording a decision to retain or remove the test.

Written by the indexing model from the issue text.