test: reuse canonical unstructured mesh assets

[kvnloo]

Follow-up shared-test-data cleanup from the direction in #4093 and pshriwise's reference branch.

Several unit tests carry duplicate unstructured-mesh assets that already exist under tests/regression_tests/unstructured_mesh/. This change points those unit consumers at the canonical regression assets and removes four duplicate unit-level copies:

• tests/unit_tests/test_mesh_tets.e
• tests/unit_tests/test_mesh_hexes.exo
• tests/unit_tests/test_mesh_dagmc_tets.vtk
• tests/unit_tests/weightwindows/test_mesh_tets.exo

The DAGMC VTK unit copy was verified content-identical to the canonical regression VTK file before removal.

The regression-suite .e / .exo files are intentionally left alone: their blobs are not byte-identical, so this PR does not assume they are interchangeable.

No production code or test behavior changes are intended.

Current head: 377be83af29831c0f74035deedcc57cfd6f43a38.
The branch is 9 commits ahead of current develop, 0 behind. Fresh fork CI is running for this amended head; upstream Actions require maintainer approval.

Fork validation: kvnloo#4

AI-assisted source review and implementation.

Credit / provenance

• @pshriwise opened DAGMC Model Test Fixutres #4093 and provided the reference direction/branch for consolidating shared test fixtures. This PR is a follow-up applying that direction to the unstructured-mesh assets.