Add test for --book-location

Author: GuillaumeGomez

tests/rustdoc/book-location.rs

@@ -0,0 +1,7 @@
+//@ compile-flags: -Zunstable-options --book-location https://somewhere.world
+
+#![crate_name = "foo"]
+
+//@ has 'foo/index.html'
+//@ has - '//*[@id="book-loc"]' 'Book'
+//@ has - '//*[@href="https://somewhere.world"]' 'Book'