Add compile flags to playground-empty test

"Run" links to the playground are not added to the generated documentation if
the unstable `--playground-url` argument is not passed to rustdoc. Therefore,
without specifying `--playground-url` as a compile-flag, the test doesn't
correctly assert that `#![doc(html_playground_url = "")]` removes playground
links.

Author: Munksgaard

src/test/rustdoc/playground-empty.rs

@@ -12,6 +12,8 @@
 
 #![doc(html_playground_url = "")]
 
+// compile-flags:-Z unstable-options --playground-url https://play.rust-lang.org/
+
 //! module docs
 //!
 //! ```