Increase test coverage

[7-of-9]

Initial focus is on e2e integration tests; note below on limitations of jest --coverage of worker_threads.

[7-of-9]

Jest is not producing coverage data for source files referenced indirectly by worker thread invocation (i.e. by new Worker()... followed by postMessage to the worker thread, per the integration tests)

This config in package.json / "jest" causes the files not directly referenced with require() to be collected in ./coverage, but as they are not referenced directly by the tests, the coverage data is still zero.
// "collectCoverageFrom": [
// "/*.{js,jsx}",
// "!/node_modules/**"
// ]

codecov/codecov-node#129