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.
