add ancient append vec test

[jeffwashington]

Problem

When we disable rewrites, we will accumulate old append vecs & slots. We use ancient append vec packing to combine those. There exist many accounts which will remain in their dead state indefinitely.
As a result, if we don't distinguish different categories of accounts where ref_count > 1, we'll have many slots which can't be packed and moved.

Summary of Changes

Add a test to prepare to distinguish accounts with refcount > 1 where there is an alive index entry with a higher slot #. Soon, accounts will be treated differently if refcount > 1 depending on if this slot's index entry for the pubkey is higher or lower than other alive entries.

This new test is very similar to the existing test_calc_accounts_to_combine_opposite. In the next pr, that test will change to test the case where we have 2 refs, and 1 is from an older slot.

Fixes #

[codecov]

Codecov Report

Merging #33762 (6200490) into master (0fcc0a0) will increase coverage by 0.0%.
Report is 7 commits behind head on master.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #33762    +/-   ##
========================================
  Coverage    81.8%    81.8%            
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  Files         806      806            
  Lines      217905   217988    +83     
========================================
+ Hits       178278   178380   +102     
+ Misses      39627    39608    -19     

[HaoranYi]

So account_with_1_ref shouldn't exist in the new_storage, right?
Can we add an assert for this?

[HaoranYi]

lgtm