Add configure option to disable running reftests
@ptomato came up with an idea, to add a configure-time option to disable running the reftests. This will help with situations like in #511, where versions of pixman/freetype/etc. differ from the ones used to generate the test reference files.
Example code at https://gitlab.gnome.org/GNOME/gjs/blob/5872749d/configure.ac#L374-384