Add unit tests for bandwidth statistics functions

We currently don't have tests for bandwidth statistics reporting code. We need to write tests for the following functions.

1. commit_max()
2. advance_obs()
3. add_obs()
4. rep_hist_bandwidth_assess()
5. rep_hist_fill_bandwidth_history()
6. rep_hist_get_bandwidth_lines()

This will be useful for legacy/trac#33617 (moved)