Fix sign error in 3D DGMultiFDSBP elixir

[jlchan]

There was a sign error in the threaded branch of the code.

This wasn't getting picked up in CI because the 2D elixir wasn't using VolumeIntegralFluxDifferencing so it wasn't triggering this branch. I've added a 3D elixir to threaded tests.

[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

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.37%. Comparing base (646f8d5) to head (3039074).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/solvers/dgmulti/sbp.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2164   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files         480      480           
  Lines       38178    38178           
=======================================
  Hits        36793    36793           
  Misses       1385     1385           

Flag	Coverage Δ
unittests	96.37% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ranocha]

I guess this came up in the kind of test that you added and gave bad results there?

[jlchan]

I guess this came up in the kind of test that you added and gave bad results there?

Yeah, I discovered it in #2150. It wasn't caught by previous CI; it should be now.