Add a test for unused_allocation lint

(how come we didn't have one already??)

Author: WaffleLapkin

tests/ui/lint/unused/unused-allocation.rs

@@ -0,0 +1,7 @@
+#![feature(box_syntax)]
+#![deny(unused_allocation)]
+
+fn main() {
+    _ = (box [1]).len(); //~ error: unnecessary allocation, use `&` instead
+    _ = Box::new([1]).len(); //~ error: unnecessary allocation, use `&` instead
+}

tests/ui/lint/unused/unused-allocation.stderr

@@ -0,0 +1,20 @@
+error: unnecessary allocation, use `&` instead
+  --> $DIR/unused-allocation.rs:5:9
+   |
+LL |     _ = (box [1]).len();
+   |         ^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-allocation.rs:2:9
+   |
+LL | #![deny(unused_allocation)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: unnecessary allocation, use `&` instead
+  --> $DIR/unused-allocation.rs:6:9
+   |
+LL |     _ = Box::new([1]).len();
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+