dex/testing: add script for two-wallet testing

[buck54321]

This is a tmux script for a 2-wallet pair that developers can use to perform manual testing on mainnet and testnet. The script aims to solve a number of problems.

1. Testing by self-trading with a single wallet has historically failed to detect some bugs.
2. Keeping your personal mainnet wallet on a release branch is advisable, so we don't want to use our primary wallet for testing.
3. For testing on mainnet, we want to use a testing server that has ultra-low lot limits and bond size, and we only want to run the nodes/markets that we're interested in testing. It wouldn't be a server suitable for public use.

[JoeGruffins]

Could add a flag for network?

Could write a small readme? It could just have whats in this pr description.

This is very helpful.

[buck54321]

Could add a flag for network?

Flags are passed through to both dexc instances.