Extract data table modules into separate files

This PR extracts the `tests` and `table_row` modules from
`data_table.rs` into separate files to improve code organization. This
is preparatory work for the upcoming column width API rework (#2 in the
series), where separating mechanical changes from logical changes will
make the review easier.

The extraction was performed using rust-analyzer's "Extract module to
file" command.

**Context:**

This is part 1 of a 3-PR series improving data table column width
handling:
1. **This PR**: Extract modules into separate files (mechanical change)
2. [#51060](https://github.com/zed-industries/zed/pull/51060) -
Introduce width config enum for redistributable column widths (API
rework)
3. Implement independently resizable column widths (new feature)

The series builds on previously merged infrastructure:
- [#46341](https://github.com/zed-industries/zed/pull/46341) - Data
table dynamic column support
- [#46190](https://github.com/zed-industries/zed/pull/46190) - Variable
row height mode for data tables

Primary beneficiary: CSV preview feature
([#48207](https://github.com/zed-industries/zed/pull/48207))

Release Notes:

- N/A