Eighteen test modules build state at import: retire the collection-guard ratchet list

[lmoresi]

tests/conftest.py now fingerprints the process state around each test module's
import and fails the run when a module moves it (#575). Eighteen modules already
do, so they are exempted by name in _KNOWN_COLLECTION_TIME_WORK — a ratchet
that stops the practice spreading, not an approval of it. This issue is the list.

Why module-level work is not merely untidy

pytest imports a module in order to collect it, so anything at module level runs
before any test, any fixture, and any isolation conftest.py provides. Two
defects reached development that way:

• Three point-locator tests fail when xdist worker count changes the file grouping #567 — a module switched the units system on at import, and the first
  module-scoped fixture in that worker built its mesh under dimensional
  coordinates. Sampled 2.9 million times too far apart; it looked like a
  parallelism bug and blocked CI parallelism for days.
• test_0050_utils.py runs two Stokes solves at module scope — pytest COLLECTION executes them and whole-directory runs hang silently #505 — a module ran two Stokes solves at import, so --collect-only sat
  inside SNESSolve looking like a silent death.

A module that builds a mesh at import has not failed yet; it has built its mesh
under whatever global state the previously collected module left behind.

The list

Fix first, because it is the only one that moves the units state
(uw.use_strict_units(False) at line 28, before three meshes and fourteen
variables — the #567 mechanism exactly):

• tests/test_0601_mesh_vector_calc.py

The rest build objects at import:

• tests/parallel/test_0765_internal_boundary_integral_mpi.py
• tests/test_0004_pointwise_fns.py
• tests/test_0005_IndexSwarmVariable.py
• tests/test_0501_integrals.py
• tests/test_0502_boundary_integrals.py
• tests/test_0504_projections.py
• tests/test_0810_amr_swarm_migration_regression.py
• tests/test_0830_mesh_adapt_variable_transfer.py
• tests/test_1000_poissonCart.py
• tests/test_1000_poissonNaturalBC.py
• tests/test_1001_poissonSph.py
• tests/test_1004_DarcyCartesian.py
• tests/test_1010_stokesCart.py
• tests/test_1011_stokesSph.py
• tests/test_1014_stokes_multigrid.py
• tests/test_1014_stokes_shell_nullspace.py
• tests/test_1050_VEstokesCart.py

The fix, per module

Move the module-level objects into a scope="module" fixture and take it as an
argument. tests/test_0050_utils.py, test_0120_data_property_access.py and
test_0130_field_creation.py were converted this way in the same PR and are the
worked examples. Delete the entry from _KNOWN_COLLECTION_TIME_WORK in the same
commit — the guard then holds that module.

Adjacent

scripts/test.sh disables tests/test_06*py wholesale ("regression tests need
validation"), which is why test_0601's import-time units flip has never been
noticed by CI: it is collected by any whole-directory run but never executed by
ours. That is a coverage gap for #570.

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