Define a subset of the testing.TB methods needed for assertions

[erutherford]

We should determine the subset of methods from the testing.TB interface we need to provide for fully functioning assertions. We should also take into consideration which methods should or shouldn't be used across goroutine boundaries. Once we've determine the subset we should update the WithTesting method and the built in assertions to use this interface.

This change should be considered a breaking change for versioning.