CI: Split Unstructured <-> DGMulti; Parabolic

[DanielDoehring]

The "unstructured_dgmulti" and "parabolic" tests run almost twice as long as the other tests (over an hour compared to more than 30 min.) This PR aims to reduce CI time by parallelizing these tests.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.42%. Comparing base (fafa540) to head (23ef740).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2234   +/-   ##
=======================================
  Coverage   96.42%   96.42%           
=======================================
  Files         487      487           
  Lines       39352    39352           
=======================================
  Hits        37942    37942           
  Misses       1410     1410           

Flag	Coverage Δ
unittests	96.42% <ø> (ø)

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[ranocha]

It looks like we just gained 15 minutes or so:

• unstructured - ubuntu-latest - x64 (pull_request) Successful in 34m
• dgmulti - ubuntu-latest - x64 (pull_request) Successful in 50m
• parabolic_part1 - ubuntu-latest - x64 (pull_request) Successful in 21m
• parabolic_part2 - ubuntu-latest - x64 (pull_request) Successful in 38m
• parabolic_part3 - ubuntu-latest - x64 (pull_request) Successful in 45m

from a bit more than an hour to 50 minutes for dgmulti and 45 minutes for parabolic stuff.

I'm fine trying to speed up CI. However, a lot of time is spent compiling stuff. What do we get when we merge the first two parabolic CI jobs?

[DanielDoehring]

We could also group 1D & 3D for the DGMulti and split this from the 2D tests.

[ranocha]

MPI on Ubuntu still takes 50 minutes, limiting the CI runtime. Would you like to split it or shall we group other jobs since 50 minutes is already fine?

Did you check whether all jobs started together or whether we need to wait for some jobs to finish before new CI resources become available?

[DanielDoehring]

We could also split the MPI tests into 2D/3D or p4est/t8code, yes.

Right, in general there are some other CI jobs running and CI times of individual Jobs differ significantly from run to run.

[DanielDoehring]

Xref: #2197