[css-color-5][editorial] add a couple of WPT

Author: svgeesus

css-color-5/Overview.bs

@@ -123,6 +123,11 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 	and
 	<var>p2</var> becomes <var>p2</var> / (<var>p1</var> + <var>p2</var>).
 
+<wpt>
+	color-mix-basic-001.tentative.html
+	color-mix-non-srgb-001.tentative.html
+</wpt>
+
 <div class="example" id="ex-mix-syntactic">
 	<!-- https://colorjs.io/notebook/?storage=https%3A%2F%2Fgist.github.com%2Fsvgeesus%2Fe37b01c449283352482d05fae52b9452 -->
 	These syntactic forms are thus all equivalent: