Add regression test for #150176

Author: Brian Campbell

tests/rustdoc/extern/pub-extern-crate-150176.rs

@@ -0,0 +1,5 @@
+//@ aux-build:pub-extern-crate.rs
+
+//@ has pub_extern_crate_150176/index.html
+//@ hasraw - 'pub extern crate inner;</code></dt>'
+pub extern crate inner;