move Linux MPI to separate CI job

[ranocha]

This CI job is failing stochastically - nearly every time in recent days. Thus, I moved it to a completely separate CI job so that the other jobs can continue running and we get coverage reports again.

We should update the required status checks before merging this PR

[sloede]

Won't this negatively affect coverage? I don't remember where the coverage for the MPI stuff was recorded...

But I like the idea to make coverage reporting work again in general!

[ranocha]

We do not test coverage with MPI on Linux at the moment, see

Trixi.jl/test/runtests.jl

Lines 22 to 29 in 5197fcb

	# In addition, it looks like the Linux GitHub runners run out of memory during the 3D tests
	# with coverage, so we currently do not test MPI with coverage on Linux. For more details,
	# see the discussion at https://github.com/trixi-framework/Trixi.jl/pull/1062#issuecomment-1035901020
	cmd = string(Base.julia_cmd())
	coverage = occursin("--code-coverage", cmd) && !occursin("--code-coverage=none", cmd)
	if !(coverage && Sys.iswindows()) && !(coverage && Sys.islinux())
	mpiexec() do cmd
	run(`$cmd -n $TRIXI_MPI_NPROCS $(Base.julia_cmd()) --threads=1 --check-bounds=yes $(abspath("test_mpi.jl"))`)

[codecov]

Codecov Report

Merging #1497 (eacb79a) into main (5197fcb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1497   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files         360      360           
  Lines       29926    29926           
=======================================
  Hits        28548    28548           
  Misses       1378     1378           

Flag	Coverage Δ
unittests	95.40% <ø> (ø)

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