Bug 1270357 Add a test hook to Subprocess.jsm r=kmag

MozReview-Commit-ID: Or2EOAL1eC

--HG--
extra : rebase_source : 05f24526d91bf7bc8ae070500fce666c4a82aa4c

toolkit/modules/subprocess/subprocess_worker_common.js

@@ -149,6 +149,10 @@ let requests = {
                                   proc => [proc.id, proc.pid]));
     return {data};
   },
+
+  waitForNoProcesses() {
+    return Promise.all(Array.from(io.processes.values(), proc => proc.exitPromise));
+  },
 };
 
 onmessage = event => {