testing: expose common struct as interface

[Olshansk]

I recently found myself doing the following in one of our test suites

func Foo[T testing.T | testing.B | testing.F](t *T) *Bar {
   // test prep stuff

	var cleanupFn func(func())
	switch any(t).(type) {
	case *testing.T:
		cleanupFn = any(t).(*testing.T).Cleanup
	case *testing.B:
		cleanupFn = any(t).(*testing.B).Cleanup
	case *testing.F:
		cleanupFn = any(t).(*testing.F).Cleanup
	default:
		log.Fatalf("Error: unsupported type %T", t)
	}
	
	cleanupFn(func(){
      // test cleanup stuff
    })

The cleanup function is common to all of these and is part of the common struct, which is embedded in each of testingT, testing.B, and testing.F.

I was wondering if common could potentially be exposed as an interface?

I'm happy to take a first stab at the PR if this is something the library mainteras would be open to, unless there's a technical reason this shouldn't be the case. Please lmk!

[seankhliao]

https://pkg.go.dev/testing#TB

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[Olshansk]

@seankhliao This is a proposal though. I'm proposing to make an interface for the common struct under testing. I didn't want to put the effort into implementation before getting a 👍 so I don't waste time implementing.

[cespare]

@Olshansk @seankhliao's message includes a link to the already-existing interface: https://pkg.go.dev/testing#TB.

[Olshansk]

That's exactly what I needed. Sorry for not RTFM the first time @seankhliao!