provisioners/ansible: try to improve unit tests

So far, I still cannot explain the cause of [these random errors](https://travis-ci.org/mitchellh/vagrant/builds/24283888) in this reusable part of the ansible provisioner unit tests,
but it is anyway safe and suitable to update the test code as following:
- use stricter regular expression matching (-l is included in --limit)
- array lengths substraction instead of array contents substraction

Let's see if the problem remains with these updates...

PS: Travis build currently fails for other reason (unit tests in guest Ubuntu/Debian).