Add a testthat the collate order is up to date

devtools::load_all may fail if there are S4 classes with inheritance
read in the wrong order.  The @include roxygen2 directive specifies the
correct order.  But this needs to be processed, and added to the collate
directive in the DESCRIPTION field before it become effective.  This
teststhat checks that load_all is reading the .R files in the correct
order