Create test for ggplot2 theme

[judith-bourque]

Issue

When modifications are made to the package, tests would allow to validate automatically that the theme still behaves as predicted.

Proposal

Create test for ggplot2 theme based on ggplot2 theme vignette in package

[judith-bourque]

• Install vdiffr
• Add differ to suggests
• Add test:

test_that("output of ggplot() is stable", {
  vdiffr::expect_doppelganger("A blank plot", ggplot())
})