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
Merge master
Replace test verification with more stable equivalent.

This verification failed in my own random test method, and I have a replacement that works, so I applied it to this test for stability.
ImmutableList<T>+Builder.AddRange perf boost.