Add make lint target running flake8
Convert assemble test to pytest
Convert expressions test to pytest
Convert extrusion_assemble_p0 test to pytest
Convert extrusion_assembly test to pytest
Convert extrusion_helmholtz test to pytest
Convert extrusion_identity test to pytest
Convert extrusion_lsw test to pytest
Convert extrusion_p0 test to pytest
Convert firedrake_extrusion_project test to pytest
Convert extrusion_rhs test to pytest
Convert extrusion_two_step test to pytest
Convert extrusion_unit_cube test to pytest
Convert extrusion_var_p0 test to pytest
Convert facets test to pytest
Convert function test to pytest
Convert helmholtz test to pytest
Convert helmholtz_nonlinear_diffusion test to pytest
Convert identity test to pytest
Convert identity_parallel test to pytest