It's unclear to me which of these kahan sums tests tests the issue you've raised with introducing kahan sums, i.e., where the kahansum returns the correct answer that a normal sum wouldn't.. I'm thinking of the following:

double a = something1, b = something2, c = something3;
answer = something4;
vector<double> vec;
vec.push_back(a); // and b and c
sum = a+b+c
EXPECT_FALSE(sum == kahanSum(vec));
EXPECT_TRUE(answer == kahanSum(vec))