Adapt tests to changes in dplyr::tbl_vars()

[romainfrancois]

The tests expected a simple character vector, and tidyverse/dplyr#4399 proposes that tbl_vars() returns a structured object of class dplyr_sel_vars with the groups attribute set to host the grouping columns.

This just adapts the test to unstructure the object.

[hadley]

I think as.character() might be more clear?

[romainfrancois]

✅ Thanks.

[hadley]

Thanks!