[css-grid-1] Add test link

Author: fantasai

css-grid-1/Overview.bs

@@ -5008,6 +5008,9 @@ Clarifications</h4>
 		<li id="change-2017-inner-outer">
 			Ensure that track sizes remain floored at zero during intrinsic sizing.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/2655">Issue 2655</a>)
+			<wpt>
+			grid-model/grid-gutters-and-tracks-001.html
+			</wpt>
 		<li id="change-2017-animation">
 			More clearly defined computed value and animation type lines of each property,
 			particularly 'grid-template-rows' and 'grid-template-columns'.