Add tests for graphical output

[mahendra-mariadassou]

Use vdiffr to add tests for graphical output. Seems to work nicely with base graphics and ggplot but less so with grobs.

[mahendra-mariadassou]

For grobs, vdffir::expect_doppelganger returns a success when run via devtools::test_file() in the console but a failure when run via the Run Tests button or via R CMD CHECK...