Introduce a Backend type for tests

[oxarbitrage]

This is a draft of what i have in mind for the refactor part of #1412

An new enum type Backend was added to TestState replacing the db_data with a backend field where the inner database type can be retrieved by methods. This should allow tests to use the framework with a WalletDb or a MockWalletDb backend.

Before making more changes, it will be good to get some feedback to see if i am in a good track or if i should rethink it.

[codecov]

Codecov Report

Attention: Patch coverage is 77.08333% with 11 lines in your changes missing coverage. Please review.

Project coverage is 61.08%. Comparing base (a1047ad) to head (2776454).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
zcash_client_sqlite/src/testing.rs	77.08%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1526      +/-   ##
==========================================
- Coverage   61.17%   61.08%   -0.09%     
==========================================
  Files         141      141              
  Lines       16645    16707      +62     
==========================================
+ Hits        10183    10206      +23     
- Misses       6462     6501      +39     

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[nuttycom]

Closing in favor of #1530