ARROW-8187: [R] Make test assertions robust to i18n

[nealrichardson]

No description provided.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8187

[nealrichardson]

@pitrou can you confirm that this resolves the failures you saw?

[pitrou]

I still get this, but other failures are fixed:

══ Warnings ════════════════════════════════════════════════════════════════════
1. Filtering on a column that doesn't exist errors correctly (@test-dplyr.R#169) - Filter expression not implemented in Arrow: not_a_col == 42; pulling data into R

══ Failed ══════════════════════════════════════════════════════════════════════
── 1. Failure: Filtering on a column that doesn't exist errors correctly (@test-
`batch %>% filter(not_a_col == 42) %>% collect()` threw an error with unexpected message.
Expected match: "object 'not_a_col' not found"
Actual message: "Cannot extract rows with an object of class NULL"
Backtrace:
  1. testthat::expect_error(...) testthat/test-dplyr.R:169:2
  9. dplyr::filter(., not_a_col == 42)
 17. arrow:::collect.arrow_dplyr_query(.data)
 20. arrow:::`[.RecordBatch`(x$.data, x$filtered_rows, colnames)
 21. arrow:::filter_rows(x, i, ...)

[nealrichardson]

Merging; dotnet release test cannot be related to this change