Allow running multiple iterations of a performance test

Add gnome-shell options:

  --perf-iters=ITERS"
    Numbers of iterations of performance module to run
  --perf-warmup
    Run a dry run before performance tests

Make a successful run of a performance test return 0 not non-zero,
and handle the difference between that and a 0-exit in normal
usage (meaning replaced) in the wrapper.

https://bugzilla.gnome.org/show_bug.cgi?id=618189

js/ui/scripting.js

@@ -203,7 +203,7 @@ function runPerfScript(scriptModule, outputFile) {
     _step(g,
           function() {
               _collect(scriptModule, outputFile);
-              Meta.exit(Meta.ExitCode.ERROR);
+              Meta.exit(Meta.ExitCode.SUCCESS);
           },
          function(err) {
              log("Script failed: " + err + "\n" + err.stack);