I have also failed to find it in Shakespeare. On a totally different mission I stumbled upon this from As You Like It, Act V. Scene I. I used the first part — “‘So so’ … so so.” — for my purposes, but thought you might enjoy the second exchange where William’s claim to wit is negated in a similar way to your quote.
Touch. ‘So so,’ is good, very good, very excellent good: and yet it is not; it is but so so. Art thou wise? 20
Will. Ay, sir, I have a pretty wit.
Touch. Why, thou sayest well. I do now remember a saying, ‘The fool doth think he is wise, but the wise man knows himself to be a fool.’