Test bulk import with all types

### Description / Background

When we added the ability to specify nullable value fields, we noticed that the test coverage for bulk import doesn't include most types. This PR comment was left unaddressed:

- https://github.com/gchq/sleeper/pull/7099#discussion_r3287239321

We'd like to test that all types can be used during bulk import, both nullable and non-nullable.

### Technical Notes / Implementation Details

For the nullable value fields, we added a test BulkImportJobDriverIT.shouldImportDataWithNullableValueField. That test currently includes value fields of two types, and only one is nullable.