Improve source-code-page.goml GUI test code

Author: GuillaumeGomez

src/test/rustdoc-gui/source-code-page.goml

@@ -2,9 +2,9 @@
 goto: file://|DOC_PATH|/src/test_docs/lib.rs.html
 // Check that we can click on the line number.
 click: ".line-numbers > span:nth-child(4)" // This is the span for line 4.
-// Unfortunately, "#4" isn't a valid query selector, so we have to go around that limitation
-// by instead getting the nth span.
-assert-attribute: (".line-numbers > span:nth-child(4)", {"class": "line-highlighted"})
+// Ensure that the page URL was updated.
+assert-document-property: ({"URL": "#4"}, ENDS_WITH)
+assert-attribute: ("//*[@id='4']", {"class": "line-highlighted"})
 // We now check that the good spans are highlighted
 goto: file://|DOC_PATH|/src/test_docs/lib.rs.html#4-6
 assert-attribute-false: (".line-numbers > span:nth-child(3)", {"class": "line-highlighted"})