paras: fix upgrade restriction signal (#4603)

Closes #3971

Read the linked issue.

Apart from that, this addresses the concern raised in this [comment] by
just adding a test. I couldn't find a clean way to reconcile a block
number delay with a PVF voting TTL, so I just resorted to rely on the
test. Should be fine for now.

[comment]: https://github.com/paritytech/polkadot/pull/4457#discussion_r770517562