In https://gerrit.wikimedia.org/r/c/mediawiki/skins/Vector/+/633828 an attempt is made to define a variable inside mediawiki.skin.variables.less that does not (yet) exist in core. The goal of this patch is to allow skins like Vector to access the variable after importing mediawiki.skin.variables.
This unexpectedly and unintuitively (at least to me) lead to a Jenkin phpunit test failure which may suggest a problem in the existing implementation.
I am not sure if this is the appropriate behaviour as I couldn't find any documentation on this ResourceLoader feature on mediawiki.org however my revised patch passed when I also added the variable to core (https://gerrit.wikimedia.org/r/c/mediawiki/core/+/635046). I am now left a little confused.
Given the specialized knowledge required for understanding ResourceLoader and the few people's heads that knowledge is in we should prioritize documenting this feature better before we use it more widely.