Prevent overriding of preconfigured casper instance in a test environment

This was a hard decision to make, but too many people just don't [read the docs](http://docs.casperjs.org/en/latest/testing.html#test-command-args-and-options) and create new casper instance in a test env, overriding the preconfigured one, therefore creating cumbersome, hard to understand situations.

Hopefully this change will inform users right from the place they read docs the most, fatal error messages.