I was asked, “What is it about evolution that testifies to God as Creator?” The implication was, nothing at all.
Evolution, like all processes driven by the laws of nature which God put into place, testifies to God as Creator. This same question could be phrased for any process seen within nature. What is it about the water cycle that testifies to God’s existence? What about the process of accretion that led to the formation of stars and galaxies testifies to God’s existence? What about the formation of snowflakes testifies to God’s existence? What about the fusion of hydrogen in the core of stars leading to supernovas and the generation of carbon and all of more complex elements required for life testifies to God’s existence? The answer is that God created the universe with an array of laws, with beautiful symmetries and constructs, that makes all of this possible. Any one of the processes described above is, frankly, amazing. So they all testify to God’s existence, but perhaps not in the way that some are wanting to see.
This question is driven by a desire to look for a specific answer that creation science proponents expect to find. It seeks a proof of God, and the use science as a tool to help in that endeavor. And not only a proof of God, but a proof of a specific interpretation of the Bible, an interpretation that leaves no room for God having used biological evolution as one of the many processes that He used for creating the universe and everything in it.
Using science as such a tool is not a fruitful exercise for two reasons. One, it is not a useful apologetic tool for spreading the gospel. For more thoughts on this specifically, see The Evidence.
Two, it leads to bad science. Many Christian teachers of science have expounded on why this is such a bad idea. We (as Christians) castigate the hard-line atheists (Dawkins, Dennett, etc) for their abuse of science to make philosophical statements about the non-existence of God. And then we turn around and abuse science to make philosophical statements about the existence of God. It goes around and around, with no end. Neither pursuit is science. For more thoughts on this, see Is Intelligent Design a Science?
As Christians, many want to know, somehow, that the creative activity within the universe requires God. Some want to know, specifically, that the diversification of life into many complex forms requires God’s direct supernatural guidance, and they want science to prove it. These are philosophical questions, not scientific questions. There is no way to prove that it’s impossible (via statistics or otherwise) for natural processes to have been the way that God diversified life. The statistical models proposed by ID scientists are far too coarse to accomplish this. For more thoughts on this, see Is Evolution Statistically Impossible? And to those who promote this view, I ask, why would God need to break His own laws in order to create millions of species? Were His laws of nature not good enough? Furthermore, the evidence for the common ancestry of life on Earth is tremendous. This includes evidence from paleontology, genetics, biogeography, and developmental biology. The evidence from endogenous retroviruses and other pseudogenes in the genomes of humans and other animals is enough to seal the deal, but it beautifully reinforces all of the other evidence that has preceded it. The evidence from pseudogenes and from developmental biology, in particular, blow the “they look similar because God used similar design patterns” argument out of the water. We, alongside other animals, inherited these shared mutations.
What Christians need to be doing is removing stumbling blocks from people’s pursuit of the Christian faith, and that is what BioLogos and similar organizations are trying to accomplish. In many cases, these stumbling blocks wouldn’t exist if it weren’t for “creation science” organizations which are trying to do the exact opposite. Not that they are trying to lay stumbling blocks, but they are promoting specific scientific-sounding explanations as foundational to the gospel message. They are shackling them to the gospel. In effect, they are adding prerequisites to a saving faith that are not actually there.
Dr. R. L. Hymers, Jr preached the following in one of his sermons, expounding on a quote from Dietrich Bonhoeffer.
In his book, Letters and Papers from Prison, Bonhoeffer asked whether mankind could cope with its problems without God. He feared that mankind might destroy the “religious premise” (God) upon which Christianity had been based for centuries. He said that Christians should be free to “share in God’s suffering in the world” following Christ’s example. He finally came to believe that science alone could not prove or disprove the existence of God. It was then that Bonhoeffer famously said,
“A God who let us prove his existence would be an idol.”
In this statement he meant that the final proof of God lies in faith, not in scientific testing. “A God who let us prove [scientifically] his existence would be an idol.” I agree with that statement. God’s existence cannot be verified, or disproved, by scientific means. God is known by revelation, given to us in the Scriptures, not by scientific testing. A god who could be tested and verified by the so-called “scientific method” would be an idol, not the real God at all, but rather the Antichrist, that malignant, Satanic creature who, in many ways, will verify his existence to a gullible and foolhardy world. 
We can and should, and must, give glory to God for Who He is and what He has done. But proving His existence through science and reason is not something God has ever asked us to do.