Add rustdoc-gui test to ensure sidebars share the same image

Author: GuillaumeGomez

tests/rustdoc-gui/sidebar.goml

@@ -199,6 +199,14 @@ assert-position-false: (".sidebar-crate > h2 > a", {"x": -3})
 drag-and-drop: ((205, 100), (108, 100))
 assert-position: (".sidebar-crate > h2 > a", {"x": -3})
 
+// Check that the mobile sidebar and the source sidebar use the same icon.
+store-css: (".mobile-topbar .sidebar-menu-toggle::before", {"content": image_url})
+// Then we go to a source page.
+click: ".main-heading .src"
+debug: true
+assert-css: ("#sidebar-button a::before", {"content": |image_url|})
+debug: false
+
 // Configuration option to show TOC in sidebar.
 set-local-storage: {"rustdoc-hide-toc": "true"}
 go-to: "file://" + |DOC_PATH| + "/test_docs/enum.WhoLetTheDogOut.html"