Add a test for useful.h.

Additionally add rotl, rotr which will be needed to implement some hashing
functions shortly.
	Change on 2014/12/15 by ctiller <ctiller@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82149559
diff --git a/build.json b/build.json
index c8f7b6b..8068bd4 100644
--- a/build.json
+++ b/build.json
@@ -436,6 +436,17 @@
       ]
     },
     {
+      "name": "gpr_useful_test",
+      "build": "test",
+      "src": [
+        "test/core/support/useful_test.c"
+      ],
+      "deps": [
+        "grpc_test_util",
+        "gpr"
+      ]
+    },
+    {
       "name": "gpr_cmdline_test",
       "build": "test",
       "src": [