prevent creation of casper instance in test env

This was a hard decision to take, but too many people just don't
read the docs and create new casper instance in a test env,
overriding the preconfigured one which creates cumbersome and
hard to understand situation.

Hopefully this change will inform users right from the place they
read docs the most, fatal error messages.
Merge pull request #728 from n1k0/prevent-casper-instance-creation-in-test-env

Prevent overriding of preconfigured casper instance in a test environment