CoW tree: tweak tests to limit at worst case, fix bug

Tweak the tests for inserts to add more objects, and use a calculated
depth limit, which actually limits at worst case. In that case, we can
get an error triggered if something is incorrect, which it was.

Add one extra height calculation, so balance factors works correctly.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>