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).

All these boxes must be checked by the AUTHOR before requesting review:

• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• If the PR is related to an issue, make sure to link it.
• The author made sure that, as a reviewer, he/she would check all boxes below.

All these boxes must be checked by the REVIEWERS before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation. Make sure to add a file documentation for each file!
• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue).

Tests

• The code is covered in an existing or new test case using Google Test.
• The code coverage of the project (reported in the CI) should not decrease. If coverage is decreased, make sure that this is reasonable and acceptable.
• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

• Should this use case be added to the github action?
• If not, does the specific use case compile and all tests pass (check manually).

Scripts and Wiki

• If a new directory with source files is added, it must be covered by the scripts/internal/find_all_source_files.sh to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example or tutorial and a Wiki article.

License

• The author added a BSD statement to doc/ (or already has one).

[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           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Davknapp]

Suggested change

	Copyright (C) 2015 the developers
	Copyright (C) 2026 the developers

[Davknapp]

we should parametrize it over some levels too, not only level 1.
Can we also start testing on level 0 and use a mesh with multiple trees?.

[Davknapp]

I am not too happy with the tree_eclass_ref-vector here. I am not sure if this is the way neighbor-information should be handled, as it causes additional memory in the size of of O(trees). I understand that is is possibly cheaper (in runtime) to use this approach, which is why I think it is ok to use it for testing.
however we should know that our tests can be used as examples to users, therefore we should at least add a comment that a decision about the tradeoff between memory/runtime has been made here.