[css-color-4] Add some gradient tests for interpolation color space

Author: svgeesus

css-color-4/Overview.bs

@@ -4356,6 +4356,12 @@ Color Space for Interpolation</h3>
 		color-mix-percents-02.html
 	</wpt>
 
+	<wpt pathprefix="css/css-images/">
+		parsing/gradient-interpolation-method-valid.html
+		parsing/gradient-interpolation-method-invalid.html
+		parsing/gradient-interpolation-method-computed.html
+	</wpt>
+
 	If the host syntax does not define what color space
 	interpolation should take place in,
 	it defaults to OKLab.