Reverse-string: add a test case with a longer string

I recently mentored a student on `ReverseString` who had started off with a naive algorithm that had quadratic complexity. It was a bit disappointing to see that in the benchmarks, the speedup was not that great. I noticed that all the sample strings in the test suite are rather short. Maybe we could add at least one longer string so that the performance gains of a better algorithm become more impressive?