Give assert.CollectT a Logf Method to allow use as mock.TestingT

[brackendawson]

From #1030: It would be nice if assert.CollectT implimented mock.TestingT so that this can work:

package kata

import (
	"fmt"
	"testing"
	"time"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/mock"
)

type myMock struct {
	mock.Mock
}

func (m *myMock) Callback(n int) error {
	return m.Called(n).Error(0)
}

func TestIfy(t *testing.T) {
	m := myMock{}
	m.Test(t)
	for i := 0; i < 10; i++ {
		m.On("Callback", i).Return(nil).Once()
	}

	go func() {
		for i := 0; i < 10; i++ {
			if err := m.Callback(i); err != nil {
				panic(fmt.Sprint("callback failed: ", err))
			}
		}
	}()

	assert.EventuallyWithT(t, func(t *assert.CollectT) {
		m.AssertExpectations(t)
	}, time.Second, time.Millisecond)
}

[dolmen]

I think that the collection process must be seriously rewritten because of #1418.

[dolmen]

I would like to get rid of assert.CollectT and just pass an opaque assert.TestingT to the condition function.

[dolmen]

The issue is more generally about mock.TestingT being a superset of assert.TestingT which means that a function being given an assert.TestingT can't pass it to mock functions.