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.
Rather than assuming that the storage instance will be attached to a VM,
just return it. Callers may attach it to a `VM` or (in the case of
`import_sb2.js`) to a `Runtime`.
- Attach the storage module to the VM for tests which load projects or
other assets.
- Move `import_sb2.js` from `test/unit/ into `test/integration` since it
now depends heavily on `scratch-storage`.
- Skip loading costumes when there is no renderer.
