Part 1: Trivial test tweak.

Differential Revision: https://phabricator.services.mozilla.com/D77636

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1642261
gecko-commit: 2e59f01bfdebdb1fc99a6bedac995713d8efb584
gecko-integration-branch: autoland
gecko-reviewers: emilio

css/css-ui/appearance-cssom-001.html

@@ -154,7 +154,7 @@
                            "statusbarpanel",
                            "tab",
                            "tab-scroll-arrow-back",
-                           "tab-scroll-arrow-forward ",
+                           "tab-scroll-arrow-forward",
                            "tabpanel",
                            "tabpanels",
                            "textfield-multiline",