Try to adjust search test to pass in CI

This test is failing on CI. I don't know why, as I cannot reproduce
locally. Perhaps it is due to the update to browser-ui-test? Or perhaps
some events from the new nav bar are delaying something?

Author: ehuss

tests/gui/search.goml

@@ -41,7 +41,7 @@ assert-css: ("#mdbook-search-wrapper", {"display": "none"})
 press-key: 's'
 wait-for-css-false: ("#mdbook-search-wrapper", {"display": "none"})
 // We ensure the search bar has the focus.
-assert: "#mdbook-searchbar:focus"
+wait-for: "#mdbook-searchbar:focus"
 // Pressing a key will therefore update the search input.
 press-key: 't'
 assert-text: ("#mdbook-searchbar", "t")