Add support for passing environment variables to the script under test

[kvas-it]

It's already possible to do it via simply modifying os.environ before running the script and then restoring it but it would be nice if script_runner.run(...) had a parameter for that. Actually run_subprocess already does: env argument will be just passed to subprocess.Popen so we just need to implement it for run_inprocess.

[a96tudor]

I'm on it