Tests: Skip html/rendering/pixel-length-attributes.html for now

This test is too slow for GCC CI.

Tests/LibWeb/TestConfig.ini

@@ -30,6 +30,7 @@ Text/input/Worker/Worker-postMessage-transfer.html
 
 ; Too slow for GCC CI
 Text/input/wpt-import/css/css-flexbox/abspos/position-absolute-001.html
+Text/input/wpt-import/html/rendering/pixel-length-attributes.html
 
 ; Skipped due to assertion failures
 Text/input/wpt-import/html/syntax/parsing/html5lib_entities01.html