Add AfterEach() method to testing contexts

Added an AfterEach() method to testing contexts. My specific use case is to clean up mock objects used in a test, like so:

```go
BeforeEach(func() {
    mockCtrl = gomock.NewController(t)
    mockDbModel = models.NewMockDbModelInterface(mockCtrl)
    handler = NewHandler(mockDbModel)
})

AfterEach(func() {
    mockCtrl.Finish() // Make sure all stubbed methods got called
})
```