Add check_ard_structure() unit test for all ard_*() functions

[ddsjoberg]

I've noticed some slight differences among the ard_*() functions with respect to column ordering, columns present, etc.

Let's add a check like the one below to all ard_*() tests.

expect_silent(
  check_ard_structure(ard_*(...))
)

Note that check_ard_structure(method=TRUE) is the default, but some ard_*() functions will not have method rows by design.