add_p() - add test names to add_p_test.<method>

[ddsjoberg]

The add_p() function adds a footnote indicating the tests performed.

When custom tests are added to the add_p() suite of tests via add_p_test.<method>(), the footnote associated with the test is listed as NA in the footnote. Update the add_p_test() functions to return not only the p-value but an attribute with the test name. The name will then be taken from the attribute.

add_p_ex2 <-
  trial %>%
  dplyr::select(response, trt) %>%
  tbl_summary(by = trt) %>%
  add_p(test = vars(response) ~ "mcnemar")