E2E test coverage

[dicarlo2]

Description

It'd be great if there was a way to get line coverage for the e2e tests, since they exercise the majority of the codebase. The difficulty comes in that the e2e tests run in multiple sub-processes, we've got the neo-one server running on one process, neo-one commands running in other processes and (potentially) a neo-one node running in another process.

At a high level the 2 main challenges are:

1. E2E tests execute logic that runs in multiple subprocesses that themselves spawn more subprocesses.
2. E2E tests run against the compiled output, so even if we fixed 1/, how do we get coverage against the original source code?

Some research:

• Using nyc with jest for subprocess coverage:
  -- Code coverage of spawned processes jestjs/jest#3190
  -- nyc+jest reports unknown for unit tests, works fine for integration tests istanbuljs/nyc#729
• Coverage
  -- Instrument prior to building: Support source maps for html report gotwarlost/istanbul#59
  -- Remap coverage using source maps: https://github.com/SitePen/remap-istanbul