Add a test for block expressions that have results. XFAILed.

Author: brson

src/test/run-pass/expr-block-box.rs

@@ -0,0 +1,11 @@
+// xfail-boot
+// xfail-stage0
+// -*- rust -*-
+
+fn main() {
+  auto x = {
+    @100
+  };
+
+  check (*x == 100);
+}