AcctIdx: tests can use index-buckets with an env var

[jeffwashington]

Problem

we have 2 code paths:

1. using disk buckets
2. not using disk buckets (in-mem only)

The 2 paths are connected (disk-buckets use in-mem for some operations). But, they are separate logic in multiple places. All tests that use accounts index ideally need to run ideally against both code paths.

Summary of Changes

default to in-mem
env var causes us to use disk buckets
Not:Randomly choose for each test to use disk buckets or not. Can we come up with a better solution other than replicating or parameterizing every test?

Fixes #

[jeffwashington]

other options:

1. plumbing with options all the way up through bank and maybe higher
2. env var where we could run cargo test twice - once for disk buckets, once for in-mem. This could be just runtime or could be all tests in the whole 'coverage' step. What about local-cluster tests? validator? ledger-tool? stable?
3. rework all accounts index tests to just run those both ways

[brooksprumo]

I think that this change is incrementally better, so can go in as-is. Long term, I think option (3) of reworking the tests to explicitly test both version would be my vote.

I'd recommend actually opening a new issue with this, then marking it as "good first issue". I wouldn't be surprised if a community member did this work.

[codecov]

Codecov Report

Merging #21494 (e3adcb5) into master (923720f) will decrease coverage by 0.2%.
The diff coverage is 80.0%.

@@            Coverage Diff            @@
##           master   #21494     +/-   ##
=========================================
- Coverage    81.6%    81.4%   -0.3%     
=========================================
  Files         511      511             
  Lines      143320   143324      +4     
=========================================
- Hits       116963   116666    -297     
- Misses      26357    26658    +301     

[t-nelson]

I think I prefer [2] as a stopgap to [3]. If we rename all of the tests to have the same prefix, we can add another CI step to do the run with the envvar set

Pull request has been modified.