Add a vdiffr dependency
It is useful to record the version number of all the packages on
which your visual test cases depend. A note containing a version
number is added to the
DESCRIPTION file for each dependency.
Dependencies on svglite and ggplot2 are automatically added. In
add_dependency() can be called in any testthat file to
manually add a dependency to a package.
A vector containing the names of the packages for which a dependency should be added.