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GH-39076: [R] Fix tests that trigger confusing dplyr warnings (#39077)

### Rationale for this change

Running our test suite results in many spurious warnings being printed that make it difficult to spot actual warnings.

### What changes are included in this PR?

The data used for specific tests involving `summarise()` was updated to not trigger the warnings.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* Closes: #39076

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