test: use make_discretization_collection

examples/advection/surface.py

examples/advection/var-velocity.py

examples/advection/weak.py

examples/euler/acoustic_pulse.py

examples/euler/vortex.py

examples/geometry.py

examples/maxwell/cavities.py

examples/wave/var-propagation-speed.py

examples/wave/wave-op-var-velocity.py

test/test_dt_utils.py

test/test_euler_model.py

test/test_grudge.py

test/test_metrics.py

test/test_mpi_communication.py

test/test_op.py

test/test_reductions.py

test/test_trace_pair.py