Coverage not generated properly

[PaulRBerg]

Tests within coverage run successfully, but the istanbul report is empty:

How to reproduce the issue:

1. Clone the feat-coverage branch of AZTEC: https://github.com/AztecProtocol/AZTEC/tree/feat-coverage
2. Cd into packages/protocol
3. Run the 0xorg/devnet container docker run -it -d --rm -p 8545:8501 -p 8546:8546 0xorg/devnet:latest /bin/bash
4. Run coverage by doing npm run coverage

Also, normal truffle tests don't work because of #4 and I cannot switch to WebSocket because of this issue: #5 (comment)

[PaulRBerg]

Closing in favour of 0xProject/0x-monorepo#1650