tests: Rename privatize.rs to deny_public_fields.rs

Privatize is no longer mentioned in the codebase, as of 8bcf36b9a5, so
rename the relevant test to avoid confusion.

tests/compiletest/plugin/compile-fail/deny_public_fields.rs

@@ -0,0 +1,18 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![allow(dead_code)]
+
+#[macro_use]
+extern crate deny_public_fields;
+
+#[derive(DenyPublicFields)]
+//~^ ERROR proc-macro derive panicked
+//~| HELP Field `v1` should not be public
+struct Foo {
+    pub v1: i32,
+    v2: i32
+}
+
+fn main() {}