Bug 1835066 - Disable a test on opt builds for now.

The test changes from the previous patch uncover an issue on shippable
builds, see referenced bug.

MANUAL PUSH: Orange fix CLOSED TREE

layout/style/test/mochitest.ini

@@ -332,6 +332,7 @@ run-if = !headless && (os == 'mac' || toolkit == 'android' || toolkit == 'gtk')
 [test_parse_rule.html]
 [test_parse_url.html]
 [test_parser_diagnostics_unprintables.html]
+skip-if = !debug # bug 1836670
 [test_pixel_lengths.html]
 [test_placeholder_restrictions.html]
 [test_pointer-events.html]