Make intrinsic-size icon render test robust to stroke-width attribute

The "Rendering an icon at its intrinsic size" scenario asserted that the
output of `wp icon render core/plus --size=none` does not contain the
substring `width=`. The core `plus` icon now carries a
`stroke-width="1.5"` attribute on the `<svg>` element, which matches that
substring even though no width/height sizing attributes are present.

Assert the absence of the concrete `width="24"` and `height="24"`
attributes instead, which is what `--size=none` is meant to leave out.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q3XHr7GsAP4YRi5y9eBznh

Author: claude

features/icon.feature

@@ -123,7 +123,11 @@ Feature: Manage WordPress SVG icons
       """
     And STDOUT should not contain:
       """
-      width=
+      width="24"
+      """
+    And STDOUT should not contain:
+      """
+      height="24"
       """
 
   @require-wp-7.1