Restore z-index on #test-body-mobile to fix header dropdown stacking (#12275)

The z-index was removed in #12126 to allow fixed-position popovers to
escape the stacking context, but this caused header menus and search
autocomplete to render behind the sticky nav-bar.

Author: lokesh

static/css/layout/index.css

@@ -12,6 +12,7 @@
   background-color: var(--white);
   border-radius: 5px;
   border: 1px solid var(--dark-beige);
+  z-index: var(--z-index-level-1);
   position: relative;
 }