arrow

ARROW-14310: [R] Make expect_dplyr_equal() more intuitive

* Renamed "input" to ".input" in test helpers to make more obvious that it's a "special" variable
* Renamed functions that don't fit the `testthat::expect_*()` pattern to prevent confusion
* Added additional documentation to helper functions

Closes #11403 from thisisnic/ARROW-14310_expect_dplyr_equal

Lead-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Nic <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>