arrow

GH-44674: [R] Fix R CMD check failure with dev testthat (#44675)

### Rationale for this change

Update assertion code in some of the R tests to be compatible with the in-development version of testthat.

### What changes are included in this PR?

Updated assertions in tests.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Fixes https://github.com/apache/arrow/issues/44674.
* GitHub Issue: #44674

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>