Remove Default derive from AccountMapEntry

[kskalski]

Problem

Default account map entry isn't really a valid value that production code creates to put into the map, there are just a few uses in tests that create artificial state.

Summary of Changes

• remove Default derive from AccountMapEntry
• add empty_for_tests guarded with cfg(test) such that tests can still use a convenience function

[kskalski]

We could possibly create entry with new(defaults...), but I think having function encapsulates this use case better and hides what exact values are used for initialization

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.0%. Comparing base (033b4a2) to head (79ed653).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8146   +/-   ##
=======================================
  Coverage    83.0%    83.0%           
=======================================
  Files         826      826           
  Lines      362192   362199    +7     
=======================================
+ Hits       300755   300761    +6     
- Misses      61437    61438    +1     

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[brooksprumo]

I agree this shouldn't drive Default.