add test for pub extern crate

Author: DebugSteven

src/test/rustdoc/auxiliary/pub-extern-crate.rs

@@ -0,0 +1,2 @@
+#![crate_name = "inner"]
+pub struct SomeStruct;

src/test/rustdoc/pub-extern-crate.rs

@@ -0,0 +1,9 @@
+// aux-build:pub-extern-crate.rs
+
+// @has pub_extern_crate/index.html
+// @!has - '//code' 'pub extern crate inner'
+// @has - '//a/@href' 'inner/index.html'
+// @has pub_extern_crate/inner/index.html
+// @has pub_extern_crate/inner/struct.SomeStruct.html
+#[doc(inline)]
+pub extern crate inner;