backstopjs-config: Ability to run register a new scenario with new viewports

[laras126]

The ?has_side_skins variation only needs to run on the desktop-xl breakpoint. Registering the URL in the current testsPaths value generates screenshots for all the breakpoints. The side skins query string should run for every template level test, and this will add quite a bit of time to running the tests and several unnecessary screenshots to the repository.