Bug 1720570 - Use backgroundImage in relative-urls.html; r=layout-reviewers,emilio

This is to work around Bug 1985880, where Firefox serializes shorthands
without making URIs absolute.

Differential Revision: https://phabricator.services.mozilla.com/D263416

Author: dminor

testing/web-platform/tests/html/semantics/scripting-1/the-script-element/css-module/relative-urls.html

@@ -11,7 +11,7 @@
     test(() => {
       const target = document.querySelector("#target");
       document.adoptedStyleSheets = [ styleSheet ];
-      let backgroundStyle = window.getComputedStyle(target).background;
+      let backgroundStyle = window.getComputedStyle(target).backgroundImage;
       assert_not_equals(backgroundStyle.indexOf("css-module/resources/image.png"), -1);
     }, "A relative URL in a CSS module should be resolved relative to the CSS file's URL, not the importing document's URL");
   </script>