Try to fix rustdoc-gui/globals.goml flakyness

Author: GuillaumeGomez

tests/rustdoc-gui/globals.goml

@@ -6,7 +6,7 @@ include: "utils.goml"
 // URL query
 go-to: "file://" + |DOC_PATH| + "/test_docs/index.html?search=sa"
 wait-for: "#search-tabs"
-assert-window-property-false: {"searchIndex": null}
+wait-for-window-property-false: {"searchIndex": null}
 
 // Form input
 go-to: "file://" + |DOC_PATH| + "/test_docs/index.html"