add an eval = "tidy" optin to check_equal()

[richierocks]

Warning: Half formed idea.

Most of the tidyverse packages now use rlang::eval_tidy(). Currently we have to set eval = FALSE in check_equal() and resort to fairly gross things like regex matching. There ought to be a way to evaluate the arguments in the context of the parent data fraem using eval_tidy(), which would make SCTs much easier to write and more robust. Since we use dplyr/ggplot2/purrr/tidyr in 90% of our R courses, this could have a big impact.