My analytic philosophy training kicks in here:
If I testify about myself, my testimony is not true. There is another who testifies on my behalf, and I know that his testimony to me is true.
We think of a statement (in logic, a proposition or a fact) as true if it corresponds to a real state of affairs. As one professor hammered into our young brains, the book is on the table is true, if and only if, the book is on the table.
But Jesus is talking about testimony and not logical propositions. And he warns us that even is own testimony about himself is not true. Testimonies, it would appear, are true, if they are relatively free from self-interst and if they are confirmed by the testimony of another. There is another who testifies on my behalf.
On the evidence of two or three witnesses the death sentence shall be executed; a person must not be put to death on the evidence of only one witness. (Deuteronomy 17:6)
And I will grant my two witnesses authority to prophesy for one thousand two hundred sixty days, wearing sackcloth. (Revelation 11:3)
But who is the one who testifies on Jesus’ behalf? We are yet to get there. But in chapter 8:
Yet even if I do judge, my judgment is valid; for it is not I alone who judge, but I and the Father who sent me. In your law it is written that the testimony of two witnesses is valid. I testify on my own behalf, and the Father who sent me testifies on my behalf. (John 8:16-18)
Speaking from a Friendly perspective, I would say it is the witness of that universal saving Light shining through our own fragile and imperfect human consciences. As we turn to it we can hear it all the better. And it rises up in confirmation.
Examine yourselves to see whether you are living in the faith. Test yourselves. Do you not realize that Jesus Christ is in you?– unless, indeed, you fail to meet the test! (2 Corithians 13:5)
But then again. Theory is always much more orderly than everyday life. And the sheep do not always recognize the shepherd’s voice. And there are beasts, with the appearances of lambs who speak with dragon’s voices.