Add `test-platform` fixture for ixmp4 tests

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR implements a few improvements related to the tests of the ixmp4-integration:

• Add a test_platform fixture in conftest
• Add a test for reading default-only runs from ixmp4
• Drop the "version" column from the meta-indicators when writing to an ixmp4 database (to avoid having a version-meta-indicator and a version-attribute of a Run) and add a test

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.1%. Comparing base (ddbb88e) to head (6ed7b56). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #832 +/- ## ======================================= + Coverage 95.0% 95.1% +0.1% ======================================= Files 64 63 -1 Lines 6134 6148 +14 ======================================= + Hits 5828 5849 +21 + Misses 306 299 -7 ```

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