You sometimes want to test concurrent behaviour like timeouts, but it
was not currently possible with our mocks.
This commits adds the following functions to Mock:
`After` will block m.Called for the given duration before returning.
```golang
func (m *Mock) After(d time.Duration) *Mock
Mock.On("MyMethod", arg1).After(1 * time.Millisecond)
```
`WaitUntil` is the primitive under `After`, and it's exposed in case you
want somebody wants more control over the returns. `Called` blocks until the w
channel is closed or receives a message.
```golang
func (m *Mock) WaitUntil(w <-chan time.Time) *Mock
Mock.On("MyMethod", arg1).WaitUntil(time.After(1 * time.Millisecond))
```
When using testify, I saw panic: assert: arguments: Error(0) failed because object wasn't correct type: {{ad6e7d77-1745-4761-6c4b-a79ab92a4a0f e6e975ef-3637-4047-436a-018ee9d2de52 %!s(int=17) <nil> <nil> %!s(bool=false) %!s(*string=<nil>) ...
when using the Error(index) function.
This commit simply changes to using %v for printing the unexpected structure in the Bool, Int, and Error shortcut methods.
