Add more verifications to ImmutableList.InsertRange perf fix.

InsertRangeRandomBalanceTest currently fails on all of my runs because VerifyBalanced finds height differences of 2 or -2.
One failing run is with rand seed: 1135977305
Call AddRange and InsertRange from corresponding Builder methods
 - This fixes failing InsertRangeRandomBalanceTest
Also refactor AddRangeBalanceTest to add random batches and verify
  balance after each