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ARROW-7360: [R] Can't use dplyr filter() with variables defined in parent scope

Closes #6014 from nealrichardson/dplyr-scope and squashes the following commits:

04d7d37ba <Neal Richardson> Update timestamp test in test-dataset.R
8017516a7 <Neal Richardson> Assert that dplyr on Arrow errors the same as regular dplyr
f692d2304 <Neal Richardson> Allow filter methods to access variables in parent scope

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>