Bug 1753565 - Disable prefers-reduced-motion for tests. r=botond

Disable the prefers-reduced-motion preference for mochitests and
web-platform-tests.

Depends on D172446

Differential Revision: https://phabricator.services.mozilla.com/D174956

Author: dlrobertson

testing/profiles/mochitest/user.js

@@ -13,3 +13,6 @@ user_pref("javascript.options.asyncstack_capture_debuggee_only", false);
 // is suppressed, synthetic click events and co. go to the old page, which can
 // be confusing for tests that send click events before the first paint.
 user_pref("nglayout.initialpaint.unsuppress_with_no_background", true);
+
+// Disable prefers-reduced-motion to ensure that smooth scrolls can be tested.
+user_pref("general.smoothScroll", true);

testing/profiles/web-platform/user.js

@@ -83,3 +83,5 @@ user_pref("remote.experimental.enabled", true);
 user_pref("privacy.partition.always_partition_third_party_non_cookie_storage", false);
 // Disable OCSP checks in WPT (webtransport triggers these occasionally)
 user_pref("security.OCSP.enabled", 0);
+// Disable prefers-reduced-motion to ensure that smooth scrolls can be tested.
+user_pref("general.smoothScroll", true);