Remove race condition in mpi testing

[andrewwinters5000]

This adds a check for MPI root and a barrier to remove a race condition in the MPI testing on the TreeMesh.

[github-actions]

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[sloede]

Thanks! Did you check whether this may also occur in other test files? Maybe https://github.com/andrewwinters5000/Trixi.jl/blob/arw-patch-1/test/test_mpi.jl as well?

[andrewwinters5000]

Thanks! Did you check whether this may also occur in other test files? Maybe https://github.com/andrewwinters5000/Trixi.jl/blob/arw-patch-1/test/test_mpi.jl as well?

Whoops, I did not. But I can add Barriers in appropriate places

[sloede]

Thanks for finding & fixing this bug!

[codecov]

Codecov Report

Attention: 220 lines in your changes are missing coverage. Please review.

Comparison is base (0799029) 74.59% compared to head (f79d859) 80.30%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/meshes/t8code_mesh.jl	58.92%	99 Missing ⚠️
src/solvers/dgsem_t8code/dg_parallel.jl	0.00%	57 Missing ⚠️
src/solvers/dgsem_t8code/containers_parallel.jl	0.00%	27 Missing ⚠️
src/callbacks_step/stepsize_dg2d.jl	0.00%	8 Missing ⚠️
src/callbacks_step/stepsize_dg3d.jl	0.00%	8 Missing ⚠️
src/callbacks_step/amr.jl	54.55%	5 Missing ⚠️
src/callbacks_step/amr_dg2d.jl	0.00%	3 Missing ⚠️
src/callbacks_step/amr_dg3d.jl	0.00%	3 Missing ⚠️
src/solvers/dgsem_p4est/dg_parallel.jl	0.00%	3 Missing ⚠️
src/callbacks_step/amr_dg.jl	0.00%	2 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1821      +/-   ##
==========================================
+ Coverage   74.59%   80.30%   +5.71%     
==========================================
  Files         452      454       +2     
  Lines       35959    36226     +267     
==========================================
+ Hits        26823    29089    +2266     
+ Misses       9136     7137    -1999     

Flag	Coverage Δ
unittests	80.30% <42.41%> (+5.71%)	⬆️

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