Use PocketIC in snsdemo

[dskloetd]

Motivation

Using PocketIC will eventually be the default, and then the only option, with dfx start.
But using it sooner will help with being able to control SNS canisters and making them run out of cycles in the test environment.

Changes

1. Pass --pocketic when starting dfx to create a snapshot.
2. Using --pocketic when starting dfx to run a snapshot.
3. Stop waiting for a checkpoint. PocketIC will write a checkpoint when it's being stopped. And waiting for one earlier will just time out.
4. Use the newest versions of the dfx extensions. nns is needed for PocketIC and sns just to keep the same version.
5. Use 0.24.3-beta.0 because older version don't support reusing the state after restarting dfx.

Tests

Tested manually starting the snsdemo snapshot from nns-dapp on Mac and Linux.