Investigate use fs mocks to speed up tests

[davewasmer]

The CommandAcceptanceTest module does a decent amount of filesystem work creating tmp dummy app copies. We should investigate:

1. How much of test latency is related to fs work done my command/app acceptance test modules?
2. If it's a significant amount, could we reduce it by introducing fs mocks?

Relevant links:

• Recipe for mocking the filesystem avajs/ava#665
• https://github.com/tschaub/mock-fs
• https://github.com/papandreou/node-mountfs

[davewasmer]

This has moved to the CLI, and at this point doesn't seem likely to be worth the effort and added complexity. Happy to revisit if anyone feels differently, but closing for now