refactor(test): separate test app interface

[cloudgray]

Description

Closes: #818

• Separate test app interface for each test with necessary keeper getter
• Let tests validate whether injected app implements specific app interface

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• tackled an existing issue or discussed with a team member
• left instructions on how to review the changes
• targeted the main branch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.23%. Comparing base (a48de71) to head (8b06dd6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #829      +/-   ##
==========================================
+ Coverage   64.94%   65.23%   +0.29%     
==========================================
  Files         317      317              
  Lines       21615    21612       -3     
==========================================
+ Hits        14037    14099      +62     
  Misses       6351     6351              
+ Partials     1227     1162      -65     

see 6 files with indirect coverage changes

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[vladjdk]

1 small nit - this is great!!!