added test type associated columns not null

[lrnzcig]

Fixes #4 for associated_columns_not_null

note that the test was broken - missing e.g. test parameters config, validations, etc

[lrnzcig]

@dividor this is the last test type that was missing in the self tests, associated_columns_not_null (issue #4)

I'm pretty sure it was broken, could you please double confirm that we are not using it in Muso?

it is not a extremely useful test, since it is a kind of special case of expression_is_true (where the expression checks for not null of a number of columns), but since it was pretty much implemented and only missing test parameters and validations, and since it is certainly not useless, I've fixed it

any thoughts?