Add support for testing.T in steps

# Summary

I built a library - a very basic version of what https://github.com/cucumber/aruba does for ruby - which is used with `go test`.

# Details

Using the standard library I have `testing.T` available to pass to the helpers - e.g. to mark the method via `t.Helper()`. Now I would love to use it with `godog`.

# Question

What would you recommend to do next? I think about adding some kind of a compatibility layer to the library. Or can we have `testing.T` on steps in "godog" as well?

Sorry the library is not (yet) open to the public.

@charlierudolph Would such support - `testing.T` - be helpful for your projects as well? I saw your request to add "gomega" as dep to "godog" in https://github.com/DATA-DOG/godog/issues/95.

No file, test, or entry point is named in the issue. Start by reviewing godog's step API and how standard-library testing.T is currently passed to helpers; clarify the desired compatibility-layer or step-signature behavior. Done means the supported approach is agreed and covered by appropriate tests.

Written by the indexing model from the issue text.