Add useful negation function tests

[theory]

• isnt_definer()
• isnt_strict()
• isnt_aggregate()

[rodo]

isnt_strict() is already done that a good point, the two others left are quite easy to code, I'll have a look

[rodo]

isnt_member_of could be useful too, don't know if I'll add in the same PR or not, depends on when the already mergeable PR will be merge ;-)

[theory]

All of these functions have been added.