add tests with vdiffr

[erblast]

https://www.tidyverse.org/articles/2019/01/vdiffr-0-3-0/

[erblast]

shiny tool does not display images correctly, this issue seems to have been reported already

does not work on windows but works on mac

[erblast]

some plots are falsely detected as being different after each render on the same system
some plots are falsely detected as being different after each render on different OS

[erblast]

plot checks that cause difficulties have been removed, will be revisited with new version of vdiffr

see release 0.1.8