todo: `demo_test.ts`

[Asher Gomez]

It should:

• Import handler() from demo.ts and use serve() from std.
• Use the signal property to close the server once tests are completed.
• Basically, ensure the demo server starts fine and endpoints are responsive, but provide 100% test coverage, if possible. In-depth testing of deno_kv_oauth's functions is not needed, as that's taken care

[Lino Le Van]

Makes sense to me.