Long-term caching testing

To test that Gatsby correctly changes its JS bundles with every type of change to site modules a user can make, here we build a site many times, making small changes between builds, to test that the bundle names change correctly.

After each run, move the public folder to public-${runNumber}.

You can easily compare run outputs with the tool Meld. meld public-1 public-2.

Runs

1. Hello world
2. Add character to src/pages/index.js
3. Add import to src/pages/index.js (import gray from "gray-percentage")
4. Add async import (import('../async'))
5. Add another async import (import("../async-2"))
6. Add character to async-2.js