Add tests for !Sized trait display

Author: GuillaumeGomez

src/test/rustdoc/sized_trait.rs

@@ -0,0 +1,17 @@
+#![crate_name = "foo"]
+
+// @has foo/struct.Bar.html
+// @!has - '//h3[@id="impl-Sized"]'
+pub struct Bar {
+    a: u16,
+}
+
+// @has foo/struct.Foo.html
+// @!has - '//h3[@id="impl-Sized"]'
+pub struct Foo<T: ?Sized>(T);
+
+// @has foo/struct.Unsized.html
+// @has - '//h3[@id="impl-Sized"]/code' 'impl !Sized for Unsized'
+pub struct Unsized {
+    data: [u8],
+}