Change the milestone test statistics to the log-log transformation

[LittleBeannie]

As indicated by Klein, J. P., Logan, B., Harhoff, M., & Andersen, P. K. (2007). Analyzing survival curves at a fixed point in time. Statistics in Medicine, 26(24), 4505-4519, the log-log transformation is more robust. So we will switch from the native approach (surv diff / std) to the log-log transformation.