add a simple test that running with hir-tree doesn't go bonkers

Akin to the existing expanded test.

src/test/run-make/hir-tree/Makefile

@@ -0,0 +1,9 @@
+-include ../tools.mk
+
+# Test that hir-tree output doens't crash and includes
+# the string constant we would expect to see.
+
+all:
+	$(RUSTC) -o $(TMPDIR)/input.hir -Z unstable-options \
+		--unpretty=hir-tree input.rs
+	grep '"Hello, Rustaceans!\\n"' $(TMPDIR)/input.hir

src/test/run-make/hir-tree/input.rs

@@ -0,0 +1,13 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    println!("Hello, Rustaceans!");
+}