testing: add Helper to the TB interface #21627
Description
As part of implementing #4899, I was going to add Helper to the TB interface. This was part of the approved design doc (https://github.com/golang/proposal/blob/master/design/4899-testing-helper.md), but I forgot to do it.
(Note that while it's not normally OK to add to exported interfaces, testing.TB has a private method specifically to allow it to be extended: https://github.com/golang/proposal/blob/master/design/4899-testing-helper.md#compatibility.)