MINOR: [R] Clean up an overly-specific test
MINOR: [R] Clean up an overly-specific test

Follow on to #11898

Closes #12005 from jonkeane/decimal-cleanup

Authored-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>
Signed-off-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>

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