Improvement: add test for t8_forest_element_neighbor_eclass

[benegee]

Describe your changes here:

I am trying to resolve #2240.

My plan was:

• pick a hybrid cmesh
• build a uniform forest
• iterate once over all trees in the forest
• for each tree record its eclass
• in the actual test, run over all trees, elements, faces
• get the neighboring tree in the forest
• get the eclass of this tree
• and compare to the output of t8_forest_element_neighbor_eclass

I had to adjust this a bit and now record the and compare to eclasses of all cmesh trees. (I was not able to move form cmesh trees to forest trees because some of them were not forest local).

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[benegee]

Not a brilliant test because the test code is actually quite close to the code being tested. Anyways, this is a start.

I think it might be useful to have a partitioned cmesh, but I was not able to produce one so far.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.59%. Comparing base (0540e17) to head (6d71d8f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2276   +/-   ##
=======================================
  Coverage   82.59%   82.59%           
=======================================
  Files         115      115           
  Lines       18545    18545           
=======================================
  Hits        15318    15318           
  Misses       3227     3227           

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