rustdoc: add assertion for missing popover div

Author: notriddle

src/test/rustdoc-gui/settings.goml

@@ -122,13 +122,15 @@ click: ".setting-line:last-child .toggle .label"
 assert-local-storage: {"rustdoc-disable-shortcuts": "true"}
 
 // Make sure that "Disable keyboard shortcuts" actually took effect.
-// The help popover won't exist yet.
+press-key: "Escape"
 press-key: "?"
+assert-false: "#help-button .popover"
 wait-for-css: ("#settings-menu .popover", {"display": "block"})
 
 // Now turn keyboard shortcuts back on, and see if they work.
 click: ".setting-line:last-child .toggle .label"
 assert-local-storage: {"rustdoc-disable-shortcuts": "false"}
+press-key: "Escape"
 press-key: "?"
 wait-for-css: ("#help-button .popover", {"display": "block"})
 assert-css: ("#settings-menu .popover", {"display": "none"})