I had a similar problem.
The solution was the method Once()
In your mock add an .Once() and repeat the mock with each result you need.
Something like this:
lib.Mock.On("method", arg).Return(test.mockError).Once()
lib.Mock.On("method", arg).Return(nil).Once()
Each mock result will be returned only once.
https://godoc.org/github.com/stretchr/testify/mock#Call.Once