Create unit test to exercise batch correction with both a batch and main effect.

[lianos]

This was a "TODO"-like item in the unit tests (test-remove-batch-effect.R), but moving it to here:

test_that("including `main` with `batch` works as expected", {
  # TODO: compare batch correction using airway dataset. It seems like including
  # `main = "cell"` doesn't have any effect, as it is being performed in the
  # FacileAnalysis,FacileAnalysis-RNAseq.Rmd vignette
  #
  # Shouldn't these two be different?
  # bc.1 <- remove_batch_effect(airway, batch = "cell")
  # bc.2 <- remove_batch_effect(airway, batch = "cell", main = "dex")
})