Add tests around SomeNotInSetConstraint.

[cmc333333]

These should cover all of the scenarios for the SomeNotInSetConstraint.

[coveralls]

Coverage increased (+4.9%) to 52.039% when pulling be5c9b0 on cmc333333:some-not-in-set-tests into 4d04c2f on python-constraint:master.

[coveralls]

Coverage increased (+4.9%) to 52.039% when pulling be5c9b0 on cmc333333:some-not-in-set-tests into 4d04c2f on python-constraint:master.

[coveralls]

Coverage increased (+4.9%) to 52.039% when pulling be5c9b0 on cmc333333:some-not-in-set-tests into 4d04c2f on python-constraint:master.

[coveralls]

Coverage increased (+4.9%) to 52.039% when pulling be5c9b0 on cmc333333:some-not-in-set-tests into 4d04c2f on python-constraint:master.

[s-celles]

Thanks @cmc333333
Code coverage is increasing (but is still quite low)