Disable position:sticky tests

Sending scroll positions to Webender is way too flaky to run the
position:sticky tests at this time. We need to disable them for now, so
that other Servo work can continue.

tests/wpt/mozilla/meta/css/css-position-3/__dir__.ini

@@ -0,0 +1 @@
+disabled: Sending multiple scrolling positions to WebRender is very flaky in the test harness