test: deflake TestProtoDowngrade
test: deflake TestProtoDowngrade

1. Don't write "nil". Not all writers handle this well.
2. Write/read an entire string.
3. Improve debuggability.
4. Move the sleep. We _don't_ need to wait for disconnect notifications.
   We _do_ need to wait for the protocol event change to propagate.

ipfs/go-libp2p,libp2p/go-libp2p,libp2p/go-libp2p