[VL] test(iceberg): Fix disabled iceberg row group bytes test

[infvg]

Previously, the row group bytes value was a hard boundary of the uncompressed value. Now the implementation causes the buffer to reach or slightly exceed the threshold.

[zhouyuan]

related #12494

[JkSelf]

@infvg @zhouyuan

This issue was introduced by a recent PR where the code writes the batch before checking if the buffer size exceeds bytesInRowGroup, causing the row group to exceed the limit.

Unlike Velox, the parquet-java reference implementation strictly ensures the row group size never exceeds bytesInRowGroup. Since Velox's current behavior deviates from this, I've opened a community issue to discuss how to define this in Velox. We should wait for that decision before making any changes here, as a fix on the Velox side would be preferred over patching it in Gluten.