Overhaul test infrastructure

[lkdvos]

Summary

• Moves test dependencies into a dedicated test/Project.toml, separating them from the main package manifest
• Replaces the monolithic test runner with ParallelTestRunner.jl, running each test file in its own worker process
• Adds --fast mode that skips AD test groups and reduces sector/scalar-type coverage for quick iteration
• Adds test/README.md documenting how to run tests, available groups, fast mode, and how to add new test files
• Updates CI to auto-discover test groups as matrix jobs; draft PRs run a reduced matrix (ubuntu + Julia 1 only) while ready PRs run the full matrix

[kshyatt]

Apart from minor comment LGTM, this will be very helpful for the Enzyme PR

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[kshyatt]

Ope, GPU failures look related...

[lkdvos]

Needs #390 first, but should then be ready.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 44 files with indirect coverage changes

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