Provide worker count env variable to processes

[nicojs]

Is your feature request related to a problem? Please describe.

When you're StrykerJS on a test suite that uses a resource (database connection, web server, file system, etc), you might want to be able to identify that you are running in a concurrent worker process and use that to choose a unique port, database, file, etc,

For example, when running StrykerJS on axios, a lot of mutants are falsely killed because all of the workers share port 4444

Describe the solution you'd like

Each worker should have a unique env variable called __STRYKER_MUTATOR_WORKER__. It should be assigned a unique number for that worker. So the first worker will get 0, second 1, etc

Describe alternatives you've considered

You can actually already misuse the active mutant for that, but I think a specific id for a worker is preferred here.

Additional context

• InfectionPHP also has this feature, they use TEST_TOKEN as env variable name.
• Node-tap also has this feature, called TAP_CHILD_ID