Summary

Allow mock expectations to be ordered

Changes

• Give each Call to be used in Mock.ExpectedCalls a []*Call which must all be satisfied first.
• Define Call.NotBefore() to allow the above to be populated.
• Check each call's requirements before processing the found expected call. This needed some existing code to be extracted to a new method Mock.checkExpectation().

Motivation

People want to check that some calls were made in a necessary order.

call1 := mockThing.On("Init").Return(nil)
call2 := mockThing.On("Do").Return(nil).NotBefore(call1)
mockThing.On("Close").Return(nil).NotBefore(call1, call2)

Related issues

Closes #741