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.