rustdoc-gui test: apply suggestions from code review

tests/rustdoc-gui/sidebar-mobile.goml

@@ -57,7 +57,8 @@ scroll-to: ".block.keyword li:nth-child(1)"
 compare-elements-position-near: (".block.keyword li:nth-child(1)", ".mobile-topbar", {"y": 544})
 
 // Now checking the background color of the sidebar.
-reload:
+// Close the sidebar menu.
+press-key: "Escape"
 show-text: true
 
 define-function: (
@@ -73,6 +74,8 @@ define-function: (
             "background-color": |background|,
             "color": |color|,
         })
+        // Make sure the sidebar is full width
+        compare-elements-size: (".sidebar", "body", ["width"])
         // Close the sidebar menu.
         press-key: "Escape"
     },