Update flow-js-testing to @onflow/[email protected]

[jribbink]

Is your feature request related to a problem? Please describe. The flow-js-testing version for unit tests generated by the playground is fairly outdated. This should be brought to date with the latest version.

Additionally, flow-js-testing has moved to a different NPM registry (@onflow/flow-js-testing). This should also be updated.

Describe the solution you'd like Update to @onflow/flow-js-testing

Additional context Linked issue in flow-js-testing repo: https://github.com/onflow/flow-js-testing/issues/141