Add GUI test to ensure there is always a theme applied if JS is disabled

Author: GuillaumeGomez

tests/rustdoc-gui/javascript-disabled.goml

@@ -3,4 +3,9 @@
 javascript: false
 
 go-to: "file://" + |DOC_PATH| + "/test_docs/struct.Foo.html"
+show-text: true
 assert-css: (".sub", {"display": "none"})
+
+// Even though JS is disabled, we should still have themes applied. Links are never black-colored
+// if styles are applied so we check that they are not.
+assert-css-false: ("a.src", {"color": "#000"})