Adding a couple simple tests around the StringBuilder

I was attempting to benchmark a few things, and see if the Cell design in StringBuilder was the best for avian.  Ultimately it does not seem worth changing StringBuilder (it's faster than array copies in avian, though slower in openJDK).

These are some small tests I was using to benchmark to test.  I thought I would go ahead and commit these, since there does not appear to be any direct tests around StringBuilder.  I did remove the code which was checking for time, and now just check for correctness.