[Testing] Use Interface over Pointer to *testing.T for Mock Expectations

[sjmtan]

By utilizing the interface, which is what the mock.AssertExpectations method uses, this allows integration with other testing libraries that might not directly expose a pointer to testing.T.

From mock/mock.go:

// AssertExpectations asserts that everything specified with On and Return was
// in fact called as expected.  Calls may have occurred in any order.
func (m *Mock) AssertExpectations(t TestingT) bool {

Gingko T in case of https://onsi.github.io/ginkgo/#integrating-with-gomock.

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