Test against ixmp4

[khaeru]

This is a minimal PR to check that the message_ix test suite runs using ixmp4 storage (i.e. without the ixmp.JDBCBackend and underlying Java code). To do this, it:

• Installs from the branch for Implement ixmp4 shim ixmp#552 that:
  • Provides a minimal 'shim' layer between the behaviour of ixmp.Platform, ixmp.TimeSeries, ixmp.Scenario and the underlying ixmp4 API.
  • Ensures that all of the behaviour of those 3 classes continues to pass the ixmp test suite when using ixmp4 storage. (message_ix and downstream/user code rely on that behaviour.)
  • Will (but does not yet, as of 2024-11-28) parametrize the ixmp.testing.test_mp fixture to yield either the current ixmp.JDBCBackend-backed test Platform, or an ixmp4-backed test Platform.
• Confirms all of the behaviour of message_ix.Scenario continues to pass the message_ix.tests suite, with either storage option. (message-ix-models and downstream/user code rely on this behaviour.)

The branch is used by iiasa/message-ix-models#257.

Implementation/to dos:

• As a consequence of parametrizing ixmp.testing.test_mp, all tests in message_ix.tests that use the test_mp fixture will run twice: once with JDBCBackend, and once with ixmp4.
• Some other tests may not be parametrized automatically by the upstream changes:
  • Identify these.
  • Parametrize them manually.
• Rebase this branch frequently.

Some things not in scope for this PR:

• Changes to the behaviour/API of message_ix.Scenario.
• Migrating large chunks of the ixmp code (including its test suite) to message_ix.

How to review

TBA

PR checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (433868d) to head (ac173e6).

Additional details and impacted files

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  Coverage   95.6%   95.6%           
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  Files         46      46           
  Lines       4337    4337           
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  Hits        4147    4147           
  Misses       190     190