Due to my weaker background for macroevolution, I believe it is harder to test it in the lab. (There may be good examples of reproducible experiments, but I am not familiar with them.) It is, however, easy to observe its effects. I've previously mentioned Darwin's finches, plant reproduction problems, and geographic isolation.
Creationism could be true. The problem is that it is difficult to test its propositions. We can observe a lot of things that seem to be the products of design, for example, there's a mantis in Indonesia that looks exactly like a flower. But, it's not scientific to attribute the result to a higher power. Doing so invokes the supernatural, which has no place in science. What's more, claiming that "God did it" is tantamount to quitting, in my opinion.
When asked to reconcile the not-perfectly-circular orbits of the planets, Newton claimed that it was the work of God. It took another scientist, Kepler, to show that the orbits are elliptical.
We may never know the answers to life, the universe, and everything. But, claiming that "God did it" and leaving it at that takes the fun out of observation, inquiry, and discovery.
I would agree with you that natural selection will change the allele frequencies in a population over time. Thats really not any different than what happens in a controlled breeding program except that survival is operating as the breeder. And I would not be surprised if that sort of variation is the explanation for Darwin’s finches.
The problem is that evolution claims the complexity of life is the result of mutation + random chance. As I discussed in my earlier posts there isn’t really any good natural explanation for the coding of the complexity of life (all the proteins and such). You reject by definition any appeal to an intelligent designer and I don’t see any natural explanation that satisfies the evidence so I guess we are at an impasse.
We've tested both micro- and macro-evolution with bacteria. Remember, "macro" refers to a change in species, not to the size of the subject. We've accomplished or observed it mainly in plants though.