Replace is.logical by isTRUE in the code

[nikosbosse]

We are often checking whether the output of a check_ function is a logical. Instead, it would be cleaner to check if the value is TRUE directly.

From the code review:

This seems a bit fragile since we wouldn't want FALSE (even though it's not supposed to pop up) to return TRUE.

test_columns_present <- function(data, columns) {
  check <- check_columns_present(data, columns)
  return(isTRUE(check))
}

Originally posted by @Bisaloo in #791 (comment)