rustdoc: add test case assertions for ArrowDown highlight first result

Author: notriddle

Diff for: src/test/rustdoc-gui/search-tab-change-title-fn-sig.goml

@@ -6,8 +6,10 @@ write: (".search-input", "Foo")
 wait-for: "#titles"
 assert-attribute: ("#titles > button:nth-of-type(1)", {"class": "selected"})
 assert-text: ("#titles > button:nth-of-type(1)", "In Names", STARTS_WITH)
+assert: "input.search-input:focus"
 // Use left-right keys
 press-key: "ArrowDown"
+assert: "#results > .search-results.active > a:nth-of-type(1):focus"
 press-key: "ArrowRight"
 wait-for-attribute: ("#titles > button:nth-of-type(2)", {"class": "selected"})
 press-key: "ArrowRight"
@@ -24,8 +26,10 @@ write: (".search-input", "-> String")
 wait-for: "#titles"
 assert-attribute: ("#titles > button:nth-of-type(1)", {"class": "selected"})
 assert-text: ("#titles > button:nth-of-type(1)", "In Function Return Types", STARTS_WITH)
+assert: "input.search-input:focus"
 // Use left-right keys
 press-key: "ArrowDown"
+assert: "#results > .search-results.active > a:nth-of-type(1):focus"
 press-key: "ArrowRight"
 wait-for-attribute: ("#titles > button:nth-of-type(1)", {"class": "selected"})
 press-key: "ArrowRight"