Coupling integral test

[int-ptr-ptr]

Description

Please describe the changes/features in this pull request.

• adds coupling_integral.hpp to unit-tests/algorithms/dim2
• provides assembly and mesh test_fixtures (currently, only 3_elem_nonconforming).
• improves prints by splitting description into name (single-line, used for test name generators) and description (multi-line verbose). Backwards compatible with prior fixtures (nonconforming_interface), but strings may want to be updated later.

Issue Number

If there is an issue created for these changes, link it here

#1342

Checklist

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• I ran the code through pre-commit to check style
• THE DOCUMENTATION BUILDS WITHOUT WARNINGS/ERRORS
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• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
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