dcou: set_accounts_hash()

[brooksprumo]

Problem

context: #32748 (comment)

This PR is a follow up #32822, which showed how to avoid creating a new function just for testing (#32822 (review)).

Summary of Changes

Conditionally make set_accounts_hash() public when the DCOU feature is enabled.

[codecov]

Codecov Report

Merging #32829 (0f9cdb5) into master (de4eee1) will increase coverage by 0.0%.
Report is 1 commits behind head on master.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #32829   +/-   ##
=======================================
  Coverage    82.0%    82.0%           
=======================================
  Files         784      784           
  Lines      211897   211902    +5     
=======================================
+ Hits       173810   173852   +42     
+ Misses      38087    38050   -37     

[ryoqun]

hehe, i was a bit hesitant to putting u64::default()s at all call sites just for the sake of removing the *_test() version. so i wanted to check that with you before creating a followup pr.

however, seems you're okay with that. :)