test: add a custom test env with a globally shared web3 provider

[fubhy]

• [x] Upgraded to Node v12
• [x] Added a custom jest test environment (based on the default node environment) that spins up the ganache fork for every test file and allocates a shared global for it removing the need to pass it around everywhere
• [x] Shared ganache configuration
• [x] Fixed a race condition in the amgu test

[fubhy]

This is the foundation for further work on https://github.com/melonproject/protocol/pull/1013