test for "setting minpoly for non-extension coeffs"
chg: hasIntegerCoefficientRing ->hasRingCoefficientRing

hasIntegerCoefficientRing handled only some special cases
and in an error prone way (relying in string representations).
hasRingCoefficientRing tests for the case where the coeffcients are not a
field which requires special care/different alg. in many cases