Update testing for plotting #70
Description
I have written a bunch of tests for spatialplanr. We are now up to 83 % coverage.
One area I am slightly unsure about it the splnr_gg_add(). @sandra-neubert can you add some more testing to it, to ensure we cover more lines. This essentially means modifying the existing test to include more of the options you added to splnr_gg_add() (contours, boundaries etc).