Add check to ensure that the line numbers are aligned with the source code

Author: GuillaumeGomez

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

@@ -11,3 +11,5 @@ assert-attribute: (".line-numbers > span:nth-child(4)", {"class": "line-highligh
 assert-attribute: (".line-numbers > span:nth-child(5)", {"class": "line-highlighted"})
 assert-attribute: (".line-numbers > span:nth-child(6)", {"class": "line-highlighted"})
 assert-attribute-false: (".line-numbers > span:nth-child(7)", {"class": "line-highlighted"})
+// This is to ensure that the content is correctly align with the line numbers.
+compare-elements-position: ("//*[@id='1']", ".rust > span", ("y"))