With 093dc915ae9ad92a3baa602eb7cb7862ca4b6734, Firefox has been updated
to the latest available version within Saucelabs. Firefox added shadow DOM support
in Firefox 63 and therefore the shadow dom test in `platform-browser` now runs as well.
This test currently fails because Firefox does not support computed style property
shorthands. In order to make this test work on Firefox now, we just switch from `border`
to `background` (because of the overhead when comparing each `top`, `bottom`, `left`, `right`-border properties)
PR Close#29518
Joey Perrott
Paul Gschwendtner
Rob Wormald