Description

Optimize several integration tests to use local fixtures instead of remote resources.

Changes:

• Move GitRepoFixture and PackageDatabase utilities to pixi_test_utils crate for reuse across test crates
• Optimize pixi_command_dispatcher tests to use local channels instead of conda-forge
• Add git fixture for multi-output-recipe to avoid remote git clones in simple_test
• Configure nextest to report slow tests with a 10 second threshold

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude Code (claude-opus-4-5-20251101)

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.