* Allow retrieving exit code from command execution
This will be helpful to derive error categories in case
an executable provides context-specific error codes.
* make sure that we always have a non 0 exit code for errors
ExecMockRunner and ShellMockRunner both needs an environment. "Extending"
here leads to "subclasses" for both cases. That is more long-winded since
it could be.
I don't understand why there should be a two dimensional array.
When dealing with envs we have normally a list containing entries like
[]string{"DEBUG=true", "HOME=/home/me"}
Having two dimensional env arrays would mean to have several alternate
environment in the tests at the same time. Don't think there is a need
for that.
