Add minimal test for box/deref.

src/Makefile

@@ -471,6 +471,7 @@ TEST_XFAILS_LLVM := $(TASK_XFAILS) \
                       $(addprefix test/run-pass/, \
                         arith-0.rs \
                         arith-2.rs \
+                        box.rs \
                         char.rs \
                         dead-code-one-arm-if.rs \
                         div-mod.rs \

src/test/run-pass/box.rs

@@ -0,0 +1,4 @@
+fn main() {
+  let @int x = @10;
+  check ((*x) == 10);
+}