Add infrastructure for compiletests

Also adds compilefail tests for some lints, closes #5646.

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

@@ -0,0 +1,16 @@
+/* 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/. */
+
+#![feature(plugin)]
+#![plugin(plugins)]
+
+
+fn main() {
+    let x = "foo".to_string();
+    //~^ ERROR str.to_owned() is more efficient than str.to_string()
+
+    let x = &x[..];
+    x.to_string();
+    //~^ ERROR str.to_owned() is more efficient than str.to_string()
+}