Add test for strikethrough in rustdoc

Author: mibac138

src/test/rustdoc/test-strikethrough.rs

@@ -0,0 +1,6 @@
+#![crate_name = "foo"]
+
+// @has foo/fn.f.html
+// @has - //del "Y"
+/// ~~Y~~
+pub fn f() {}