Add test driver with the notion of perf tests.

benchmarks/v8.json

 {
   "path": ["."],
   "main": "run.js",
   "run_count": 2,
   "results_regexp": "^%s: (.+)$",
-  "benchmarks": [
+  "tests": [
     {"name": "Richards"},
     {"name": "DeltaBlue"},
     {"name": "Crypto"},
     {"name": "RayTrace"},
     {"name": "EarleyBoyer"},
     {"name": "RegExp"},
     {"name": "Splay"},
     {"name": "NavierStokes"}
   ]
 }