Make "testdouble" dependency optional in MockProcess class

[ro0gr]

Seems like mock-process caused some inconveniences due to its testdouble dependency.

Actually we need testdouble only in will-interrupt-process unit tesst. We have nothing to do with testdouble in any other scenarios.

Also previously I thought it was a good idea to implement MockProcess.exit() as a testdouble.function(). Unfortunately it wasn't. This approach isn't able to replicate a real process.exit() behavior and leads to dishonest tests when we test cli/cli for example.
That is the reason why I changed MockProcess.exit() to throw an exception.

Known packages who was forced to install testdouble due to ember-cli@2.16 upgrade:

• ember-cli-blueprint-test-helpers
• ember-cli-page-object
• ...

[Turbo87]

thanks @ro0gr!