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GH-38293: [R] Fix non-deterministic duckdb test (#38294)

### Rationale for this change

The test fail with the latest version of duckdb (0.9.1).

### What changes are included in this PR?

The test was changed so that it did not depend on non-deterministic behaviour. We sort all of the other expectations involving a group_by to avoid this problem...we hadn't changed this one yet because it didn't fail in any previous version of duckdb.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* Closes: #38293

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>