Add performance test comparison of Scenario.clone()

[glatterf42]

@meksor and @khaeru: here are some tests that I used locally to start investigating why Scenario.clone() is slower on IXMP4Backend than on JDBC. This investigation is not finished, I just mean to make these files available to you :)
Not sure if we want to have these tests around in the future, so please feel free to do with this PR whatever you see fit.

How to review

TBD

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[khaeru]

Excellent, thanks!

[codecov]

Codecov Report

❌ Patch coverage is 0% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.7%. Comparing base (9448027) to head (e9e87cc).

Files with missing lines	Patch %	Lines
ixmp/tests/performance/clone.py	0.0%	45 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #620     +/-   ##
=======================================
- Coverage   98.3%   97.7%   -0.7%     
=======================================
  Files         51      52      +1     
  Lines       6624    6669     +45     
=======================================
  Hits        6518    6518             
- Misses       106     151     +45     

Files with missing lines	Coverage Δ
ixmp/tests/performance/clone.py	0.0% <0.0%> (ø)

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