Add unit tests for UpdateStatisticsAction functionality.

[slfan1989]

Is your feature request related to a problem or challenge?

Added tests for setting and removing statistics files in UpdateStatisticsAction.

• Tests cover scenarios for:

1. Setting a single statistics file.
2. Ensuring no statistics are set.

Describe the solution you'd like

This is my first time contributing to the Iceberg-rust community. This PR adds two unit tests, and I hope it will be accepted.

Willingness to contribute

Yes