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ARROW-17689: [R] Implement dplyr::across() inside group_by() (#14122)

Because the handling of the case `.add = TRUE` and the `add` argument have been changed, test cases for these are also added.

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>