Fix #2737: set process.execPath for cluster module

See #2737.

`process.execPath` must be set to the location of the CoffeeScript
interpreter when running a CoffeeScript file, in order to allow the
`cluster` module (as well as `child_process.fork`) to know how to run
workers written in CoffeeScript.

This also adds in a test to ensure that this functionality continues to
work.