Add GUI test for scrollbar position on source code page

Author: GuillaumeGomez

tests/rustdoc-gui/source-code-page-code-scroll.goml

@@ -0,0 +1,8 @@
+// Checks that the scrollbar is visible on the page rather than the code block.
+goto: "file://" + |DOC_PATH| + "/src/test_docs/lib.rs.html"
+size: (800, 1000)
+// "scrollWidth" should be superior than "clientWidth".
+assert-property: ("body", {"scrollWidth": 1047, "clientWidth": 800})
+
+// Both properties should be equal (ie, no scroll on the code block).
+assert-property: (".example-wrap .rust", {"scrollWidth": 933, "clientWidth": 933})