suite: allow Suite methods to take *testing.T parameter

[grongor]

Summary

Allow suite methods to take *testing.T parameter (as normal tests do). This is BC, and we are already messing with reflection there, so no penalties either.

Changes

Extend allowed suite method signature to include optional *testing.T.
Fail with a friendly error when the signature doesn't match (before it panicked).

Motivation

There are many places where you need t, like for example for mocks and other testing libraries. And typing s.T() get's tedious after a while, and is completely avoidable at no cost :)

type MyTestSuite struct {
	suite.Suite
}

func (s *MyTestSuite) TestLorem(t *testing.T) {
	s.Same(t, s.T())

	mockA := mocks.NewMockA(t)
	mockB := mocks.NewMockB(t)
	mockIHaveTooManyMocks := mocks.NewMockMyLife()

	testStuff(mockA, mockB, mockIHaveTooManyMocks)
}

func (s *MyTestSuite) TestSimple() {
	s.NotEqual("this is", "backwards compatible")
}