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GH-33777: [R] Nightly builds failing due to dataset test not being skipped on builds without datasets module (#33778)

The minimal builds were failing due to a new test which requires the datasets module; I've just skipped this test on builds where this module isn't installed.

* Closes: #33777

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>