test: don't use process.exit to end tests

Newer versions of `tap` run more asynchronously, so sometimes using `process.nextTick(process.exit)`
to end a test would prevent the test from completing correctly. Removing all instances of
`process.nextTick(process.exit)` put tests into three categories:
* the test still worked correctly -- no fixup needed.
* the test would hang because the VM's `_steppingInterval` was keeping
  Node alive. These tests call a new `quit()` method which ends the
  stepping interval.
* the `load-extensions` test needed special attention because the "Video
  Sensing" extension starts its own loop using `setTimeout`. I added a
  `_stopLoop()` method on the extension and directly call that from the
  test. I'm not completely happy with this solution but anything more
  general would likely require a change to the extension spec, so I'm
  leaving that as a followup task.

test/integration/sensing.js

@@ -15,8 +15,8 @@ test('sensing', t => {
     vm.on('playgroundData', e => {
         const threads = JSON.parse(e.threads);
         t.ok(threads.length > 0);
+        vm.quit();
         t.end();
-        process.nextTick(process.exit);
     });
 
     // Start VM, load project, and run