Tests: adjust to use new-style actx fixture

test/test_array.py

test/test_chained.py

test/test_connection.py

test/test_discretization.py

test/test_firedrake_interop.py

test/test_interop.py

test/test_mesh.py

test/test_meshmode.py

test/test_modal.py

test/test_partition.py

test/test_refinement.py

test/test_visualization.py