[css-ui] Correct property name

[jugglinmike]

The test for the appearance property mistakenly referred to the vendor
prefixed version, -webkit-appearance. In addition to contradicting the
test name and metadata, this caused the test intended for
-webkit-appearance to reference the non-existent property
-webkit--webkit-appearance.

Correct both tests, using the suite's build process to generate the
-webkit-appearance test from the corrected version of the appearance
test.

---

Recent research has demonstrated that the menulist-button value is necessary for web compatibility.

This seems to be the only instance of this particular mistake:

$ git rev-parse HEAD
48af14aaebdbcaf90bc21d07965f7c16661f8d90
$ git grep -E 'moz-+moz'
$ git grep -E 'webkit-+webkit'
css/css-ui/webkit-appearance-menulist-button-002.html: #container > #drop-down-select { -webkit--webkit-appearance: none; -webkit--webkit-appearance: menulist-button; }
$

[zcorpan]

Whooops. Good catch. This is a disadvantage of copy-paste programming.