GH-48145: [R] Update to testthat 3.3.0 and use its expect_r6_class()

Generated-by: Claude

Author: thisisnic

r/DESCRIPTION

@@ -68,7 +68,7 @@ Suggests:
     stringi,
     stringr,
     sys,
-    testthat (>= 3.1.0),
+    testthat (>= 3.3.0),
     tibble,
     tzdb,
     withr

r/tests/testthat/helper-expectation.R

@@ -24,11 +24,6 @@ expect_equal_data_frame <- function(x, y, ...) {
   expect_equal(as.data.frame(x), as.data.frame(y), ...)
 }
 
-expect_r6_class <- function(object, class) {
-  expect_s3_class(object, class)
-  expect_s3_class(object, "R6")
-}
-
 #' Mask `testthat::expect_equal()` in order to compare ArrowObjects using their
 #' `Equals` methods from the C++ library.
 expect_equal <- function(object, expected, ignore_attr = FALSE, ..., info = NULL, label = NULL) {