Run node tests in separate process

[jason0x43]

This issue was split off from #780.

Intern 4 currently lets node be configured as if it were an remote environment (e.g., it must be specified in the environments config property). However, a more thorough implementation of this approach would involve starting new Node processes to run unit tests as if they were remote browsers. This could provide a number of benefits, such as:

• It would be more apparent in a config which environments tests were being run in
• Tests could be run in multiple versions of Node
• Any solution developed for partitioning test sets and running groups in parallel would implicitly work for both Node and browser tests