require: Channel assertions

[tallclair]

I've found the following test helpers useful (especially when paired with testing/synctest):

func requireBlocked[T any](t *testing.T, ch <-chan T) {
	t.Helper()
	select {
	case r := <-ch:
		t.Fatalf("Channel receive should have blocked, but got: %v", r)
	default:
		// OK.
	}
}

func requireUnblocked[T any](t *testing.T, ch <-chan T) (r T) {
	t.Helper()
	select {
	case r = <-ch:
	default:
		t.Fatalf("Channel receive should not have been blocked")
	}
	return
}

These seem like they could be useful assertions to include.

[dolmen]

The signature of those functions doesn't match the assertions in packages assert and require.

Do you have a design that would better fit?

[dolmen]

What about checking len(ch) vs cap(ch)

require.Len(t, ch, cap(ch)) // requireBlocked
require.Less(t, len(ch), cap(ch))  // requireUnblocked

Those checks don't consume a value from the channel.