rustdoc: fix test case for custom themes

Author: notriddle

tests/run-make-fulldeps/rustdoc-themes/foo.rs

@@ -1,4 +1,4 @@
 // @has test.css
 // @has foo/struct.Foo.html
-// @has - '//link[@rel="stylesheet"]/@href' '../test.css'
+// @has - '//*[@id="rustdoc-vars"]/@data-themes' 'test'
 pub struct Foo;