emphasize unit testing yarn test --watch for troubleshooting

[dckc]

deploy --watch (Agoric/dapp-agoric-basics#62) is a cool idea, but when things aren't working, we want devs to reach for unit testing first.

yarn test --watch is already mature. In Kate's Dec 2020 workshop, she uses it the whole time.

Maybe we need to advertise yarn test --watch on every path leading into our stuff:

• the getting started tutorial and video
• READMEs of all 3 dapps
• ... where else?

[dckc]

Somebody mentioned learning to use debugger statements from MarkM's session in Apr 24 office hours. Maybe I/we should link that from a few of these places too.

or maybe not... I'm struggling to even find the relevant part of the session. can't expect others to find it. Better to record a new short clip.