specify 'norbs' on lattice creation in all tests

kwant/continuum/tests/test_discretizer.py

kwant/linalg/tests/test_mumps.py

kwant/physics/tests/test_dispersion.py

kwant/physics/tests/test_leads.py

kwant/physics/tests/test_noise.py

kwant/solvers/tests/_test_sparse.py

kwant/tests/test_builder.py

kwant/tests/test_comprehensive.py

kwant/tests/test_lattice.py

kwant/tests/test_operator.py

kwant/tests/test_plotter.py

kwant/tests/test_system.py

kwant/tests/test_wraparound.py