Bug 1361054 - use registerCleanup in scratchpad browser tests;r=ochameau

AFAICT SimpleTest.registerCleanup is never called here. And it introduced a leak
in the session scratchpad test I want to move to this test suite. I couldn't really
understand where the leak was coming from, but using registerCleanup (which _is_
called) fixes it.

MozReview-Commit-ID: 2VE7wHxy066

--HG--
extra : rebase_source : b2e7f62a727535912be55c68d6f8b0932e9f1414

devtools/client/scratchpad/test/head.js

@@ -18,7 +18,7 @@ const promise = require("promise");
 var gScratchpadWindow; // Reference to the Scratchpad chrome window object
 
 flags.testing = true;
-SimpleTest.registerCleanupFunction(() => {
+registerCleanupFunction(() => {
   flags.testing = false;
 });