Remove sandbox backend tests

[lucasmrod]

Removing some Sandbox tests and some old proposals that are not valid anymore.

(As agreed in weekly backend.)

[mostlikelee]

In case sandbox comes back in the future, is it as simple as reverting this commit?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.09%. Comparing base (98c0bd8) to head (b1fd8d7).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22510      +/-   ##
==========================================
- Coverage   65.22%   65.09%   -0.14%     
==========================================
  Files        1495     1495              
  Lines      116940   116940              
  Branches     3578     3578              
==========================================
- Hits        76279    76126     -153     
- Misses      33581    33731     +150     
- Partials     7080     7083       +3     

Flag	Coverage Δ
backend	66.36% <ø> (-0.15%)	⬇️

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[lucasmrod]

In case sandbox comes back in the future, is it as simple as reverting this commit?

If it comes back it may be in a different form?, but yes, reverting the commit should work.

[lucasmrod]

Failing test is a flaky test and codecov seems was successful but is stuck on the codecov side.