After these changes the Launcher will delegate more of how a test
is run down to the actual implementation of the runtime platform
(e.g. local, AWS Lambda, AWS Fargate or something else).

It will no longer manage workers directly via createWorker(),
prepareWorker() etc.

The only concerns handled in Launcher now are to:

- Initialize a runtime platform implementation
- Subscribe to events emitted by the platform
  - Worker-level events such as 'stats' or 'phaseCompleted' and
    aggregate metric reports from multiple workers

This was previously done at Launcher level, but
has now been delegated to platform implementation

A worker may encounter an error that does not necessarily require
stopping the execution of a test script, e.g. a local worker may not
be able to load a plugin. The worker can communicate such errors
by setting a "level" property on the workerError message.

This change takes level into account for determining whether the
worker has stopped running or not.

Previous version of this code has a bug, where the stoppedError
state assigned to the worker [1] would get overridden elsewhere
meaning that the condition that determined whether all workers
exited [2] would not evaluate to true as expected. It also had
the same issue of not taking level into account, meaning that
code that handled loading plugins in local worker threads[3] should
have stopped the execution of a test early, but didn't.

1. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/launch-platform.js#L79
2. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/launch-platform.js#L242
3. https://github.com/artilleryio/artillery/blob/1f6e1c6d95d943f1ee6e5ef5b422f8ea70c24b85/packages/artillery/lib/platform/local/worker.js#L160-L167

Increase the duration of the test to make sure that at least one
intermediate report is produced before the workers are terminated
due to OOM. An assertion in an e2e test relies on having at least
one intermediate report produced:

https://github.com/artilleryio/artillery/blob/5a01e3632c4b6959ad94299bd7caf37397331c6b/packages/artillery/test/cloud-e2e/fargate/memory.test.js#L22