Improve test coverage of variable handling for included tasks

Hello,

This is my first pull request on ansible and on github at the same time I hope it is both correct and not too "minimal" ;-)

Currently the test regarding included tasks does not check that the values of the variables are correctly handed to the task; this commit tries to improve the coverage.

I made this commit while trying to locate/patch a bug regarding the sudo_user parameter (more on this in a later pull request) and thought it could improve the overall test coverage of ansible.

Tell me what you think