Clone Scenario.scheme on all clone() calls

[glatterf42]

Some of the tests on iiasa/message_ix#894 fail because the .scheme property of a Scenario is not cloned when calling clone() on an IXMP4Backend (and instead set to "IXMP4-MESSAGE"), which message_ix doesn't expect. It expects "MESSAGE", but we can't hardcode that either because some ixmp tests expect something else (and we generally want to avoid hardcoding things, of course).

So this PR adjusts the clone() call on IXMP4Backend to also clone the .scheme.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation. Just a small internal fix.
• [ ] Update release notes. Just a small internal fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.4%. Comparing base (c07d3f3) to head (3f39373).
⚠️ Report is 81 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #571     +/-   ##
=======================================
- Coverage   98.4%   98.4%   -0.1%     
=======================================
  Files         50      50             
  Lines       5935    5941      +6     
=======================================
+ Hits        5841    5846      +5     
- Misses        94      95      +1     

Files with missing lines	Coverage Δ
ixmp/backend/ixmp4.py	90.2% <ø> (ø)
ixmp/tests/core/test_scenario.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[khaeru]

I think this is the kind of small nuance of ixmp behaviour that previously lacked tests. If it had been tested, we would have caught this in #552. So, added the test that was missing.

Otherwise good to go once CI passes.