Add test for source code clickable line number

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

+goto: file://|DOC_PATH|/../src/test_docs/lib.rs.html
+// Check that we can click on the line number.
+click: (40, 224) // This is the position of 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: (".line-numbers > span:nth-child(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-false: (".line-numbers > span:nth-child(3)", "class", "line-highlighted")
+assert: (".line-numbers > span:nth-child(4)", "class", "line-highlighted")
+assert: (".line-numbers > span:nth-child(5)", "class", "line-highlighted")
+assert: (".line-numbers > span:nth-child(6)", "class", "line-highlighted")
+assert-false: (".line-numbers > span:nth-child(7)", "class", "line-highlighted")