go-testing-interface is a Go library that exports an interface that *testing.T implements as well as a runtime version you can use in its place.

The purpose of this library is so that you can export test helpers as a public API without depending on the "testing" package, since you can't create a *testing.T struct manually. This lets you, for example, use the public testing APIs to generate mock data at runtime, rather than just at test time.

For usage and examples see the Godoc.

Given a test helper written using go-testing-interface like this:

import "github.com/mitchellh/go-testing-interface"
func TestHelper(t testing.T) {
    t.Fatal("I failed")
}

You can call the test helper in a real test easily:

import "testing"
func TestThing(t *testing.T) {
    TestHelper(t)
}

You can also call the test helper at runtime if needed:

import "github.com/mitchellh/go-testing-interface"
func main() {
    TestHelper(&testing.RuntimeT{})
}

*Why would I call a test helper that takes a testing.T at runtime?

You probably shouldn't. The only use case I've seen (and I've had) for this is to implement a "dev mode" for a service where the test helpers are used to populate mock data, create a mock DB, perhaps run service dependencies in-memory, etc.

Outside of a "dev mode", I've never seen a use case for this and I think there shouldn't be one since the point of the testing.T interface is that you can fail immediately.