Improve @metamask/snaps-cli unit tests

[rekmarks]

Following MetaMask/snaps-cli#74, there are still some unit test improvements to make:

• Achieve 100% branch coverage for all files
• Test the built evalWorker file
• Test the yargs-generated argv object for all commands
  • See TODO in cli.test.ts

[rekmarks]

@hmalik88 how are we doing with this?

[hmalik88]

Haven't got to this issue yet, but it's partially blocked by #556 as that will change worker implementation possibly. We're at 100% coverage minus eval-worker.js, I need to have a chat with Kris from Agoric about testing lockdown (I had a method when working on execution-environments but I'm not sure if that's proper). Will be able to address after NFT NYC week.