🐛 Keep GoogleTest discovery artifacts in the build tree

[burgholzer]

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Description

CMake 4.3 writes a cmake_test_discovery_*.json bookkeeping file to the directory passed to gtest_discover_tests(WORKING_DIRECTORY ...). Our test helpers passed source directories there because the same argument also controls the runtime working directory of each discovered test. As a result, configuring and building tests could dirty the source tree.

Run GoogleTest discovery from the current binary directory so the bookkeeping file remains a build artifact. Preserve the existing runtime working directory by setting the discovered tests' WORKING_DIRECTORY property separately. The property name is deferred through a generator expression so CMake's argument parser does not interpret it as a second discovery option.

This is extracted from #1912 because it is independent of the QDMI configuration work.

Validation

• Configured and built representative targets with CMake 4.3.2.
• Confirmed that no cmake_test_discovery_*.json files appear outside build/.
• Confirmed that the ordinary helper still runs a FoMaC test from its source directory.
• Confirmed that the custom-working-directory helper still runs an IR test from its configured build directory.
• uvx nox -s lint
• git diff --check

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality. (Existing tests exercise both helper variants.)
• I have updated the documentation to reflect these changes. (No user-facing documentation change is needed.)
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals. (Not user-facing.)
• I have added migration instructions to the upgrade guide (if needed). (No migration is needed.)
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks. (Local checks pass; CI is pending.)
• I have reviewed my own code changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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