Add tests

tests/compile-fail/used_underscore_binding.rs

@@ -0,0 +1,22 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#[deny(used_underscore_binding)]
+
+fn main() {
+    let foo = 0u32;
+    prefix_underscore(foo); //should fail
+    non_prefix_underscore(foo); //should pass
+    unused_underscore(foo); //should pass
+}
+
+fn prefix_underscore(_x: u32){
+    println!("{}", _x + 1); //~Error: Used binding which is prefixed with an underscore
+}
+
+fn non_prefix_underscore(some_foo: u32) {
+    println!("{}", some_foo + 1);
+}
+
+fn unused_underscore(_foo: u32) {
+    println!("{}", 1);
+}