Switch to TestItems.jl setup

[JoshuaLampert]

This is a first draft to for moving to a TestItems.jl based testing workflow, see #3092. Currently, it is mainly to see how CI reacts to it.

TODO:

• Basic setup with only one test file migrated to TestItems.jl
• Investigate splitting tests for TRIXI_TEST using tags
• Migrate threaded tests
• Migrate KernelAbstractions tests
• Migrate MPI tests
• Migrate CUDA/AMDGPU tests
• Migrate all remaining tests
• Clean up and polish AI generated code

Issues:

• DoubleFloats not available in test elixir_advection_doublefloat.jl of test/test_tree_1d_advection.jl (fixed by escaping l2 and linf in TrixiTest.jl).
• cleanups at the end of file? (cleaning the output dir has been moved to runtests.jl, such that it is always cleaned up at the beginning and at the end of each test run)

[github-actions]

Review checklist

This checklist is meant to assist creators of PRs (to let them know what reviewers will typically look for) and reviewers (to guide them in a structured review process). Items do not need to be checked explicitly for a PR to be eligible for merging.

Purpose and scope

• The PR has a single goal that is clear from the PR title and/or description.
• All code changes represent a single set of modifications that logically belong together.
• No more than 500 lines of code are changed or there is no obvious way to split the PR into multiple PRs.

Code quality

• The code can be understood easily.
• Newly introduced names for variables etc. are self-descriptive and consistent with existing naming conventions.
• There are no redundancies that can be removed by simple modularization/refactoring.
• There are no leftover debug statements or commented code sections.
• The code adheres to our conventions and style guide, and to the Julia guidelines.

Documentation

• New functions and types are documented with a docstring or top-level comment.
• Relevant publications are referenced in docstrings (see example for formatting).
• Inline comments are used to document longer or unusual code sections.
• Comments describe intent ("why?") and not just functionality ("what?").
• If the PR introduces a significant change or new feature, it is documented in NEWS.md with its PR number.

Testing

• The PR passes all tests.
• New or modified lines of code are covered by tests.
• New or modified tests run in less then 10 seconds.

Performance

• There are no type instabilities or memory allocations in performance-critical parts.
• If the PR intent is to improve performance, before/after time measurements are posted in the PR.

Verification

• The correctness of the code was verified using appropriate tests.
• If new equations/methods are added, a convergence test has been run and the results
  are posted in the PR.

Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.60%. Comparing base (c9f3657) to head (8b7d5a3).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3096       +/-   ##
===========================================
- Coverage   96.88%   29.60%   -67.28%     
===========================================
  Files         647      647               
  Lines       50035    49886      -149     
===========================================
- Hits        48475    14766    -33709     
- Misses       1560    35120    +33560     

Flag	Coverage Δ
unittests	29.60% <ø> (-67.28%)	⬇️

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