Only run the at_exit behaviors if the current PID matches the PID that called SimpleCov.start

[coderanger]

This handles the case where a test calls fork at some point, the new process shouldn't try to run coverage formatters or check the minimum coverage level.

This resolves #371.

[coderanger]

Not sure whats up with that test failure, can't repro it locally.

[colszowka]

Great, thanks. Don't worry about the failing tests, we had an order issue there, see #362 - which should be resolved now that I've merged #376.

[grosser]

FYI this breaks using forking test runners with code coverage for example https://github.com/grosser/codeclimate_batch + https://github.com/grosser/forking_test_runner

not a big issue, just wanted you to be aware that this is a usecase / tell me if this looks wrong and I should be doing something else

fork do
  SimpleCov.pid = Process.pid
  ... run test ...
end