Fixes block form of #start + add specs

[jpcamara]

• Because the ensure calls _stop instead of stop, the block form produces invalid json

• Perfetto is pretty permissive about this and still accepts the file - but other tools break and JSON parsing is broken

• Also adds first specs for the project, covering a few different scenarios 🎉

[ivoanjo]

Interestingly, even without MN, some of the tests break on 3.3 in CI . It's no worse than it was before so at least now we know about it :D