Need to remove 'using namespace std' from tests

[bangerth]

In C++20, there now is a namespace std::numbers: https://en.cppreference.com/w/cpp/symbol_index/numbers. Since we have a namespace dealii::numbers and we typically do using namespace dealii in the tests, every reference to numbers becomes ambiguous if a test also has a using namespace std and fails to compile. Unfortunately, we have quite a lot of those:

> grep -r 'using namespace std' tests/
tests/bits/map_dofs_to_support_points_hp_01.cc:using namespace std;
tests/bits/count_dofs_per_component_hp_02.cc:using namespace std;
tests/bits/count_dofs_per_component_hp_01.cc:using namespace std;
tests/bits/map_dofs_to_support_points_hp_02.cc:using namespace std;
tests/base/polynomials_tensor.cc:using namespace std;
tests/base/polynomials_bdm_02.cc:using namespace std;
tests/base/polynomials_bdm_01.cc:using namespace std;
tests/base/polynomials_rt.cc:using namespace std;
tests/trilinos/mg_transfer_prebuilt_01.cc:using namespace std;
tests/multigrid/renumbering_05.cc:using namespace std;
tests/multigrid/transfer_select.cc:using namespace std;
tests/multigrid/boundary_01.cc:using namespace std;
tests/multigrid/transfer_system_04.cc:using namespace std;
tests/multigrid/transfer_05.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_02.cc:using namespace std;
tests/multigrid/transfer_06.cc:using namespace std;
tests/multigrid/transfer_01.cc:using namespace std;
tests/multigrid/renumbering_03.cc:using namespace std;
tests/multigrid/mg_output_neumann.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_05.cc:using namespace std;
tests/multigrid/transfer_block.cc:using namespace std;
tests/multigrid/mg_output_dirichlet.cc:using namespace std;
tests/multigrid/transfer_04b.cc:using namespace std;
tests/multigrid/mg_level_obj_01.cc:using namespace std;
tests/multigrid/constrained_dofs_03.cc:using namespace std;
tests/multigrid/transfer_03.cc:using namespace std;
tests/multigrid/transfer_system_03.cc:using namespace std;
tests/multigrid/transfer_02.cc:using namespace std;
tests/multigrid/transfer_compare_01.cc:using namespace std;
tests/multigrid/renumbering_01.cc:using namespace std;
tests/multigrid/transfer_prebuilt_03.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_03.cc:using namespace std;
tests/multigrid/constrained_dofs_01.cc:using namespace std;
tests/multigrid/transfer_04a.cc:using namespace std;
tests/multigrid/transfer_04.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_01.cc:using namespace std;
tests/multigrid/renumbering_07.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_09.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_08.cc:using namespace std;
tests/multigrid/renumbering_04.cc:using namespace std;
tests/multigrid/dof_01.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_06.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_07.cc:using namespace std;
tests/multigrid/renumbering_06.cc:using namespace std;
tests/multigrid/count_01.cc:using namespace std;
tests/multigrid/sparse_matrix_collection_01.cc:using namespace std;
tests/multigrid/renumbering_08.cc:using namespace std;
tests/multigrid/constrained_dofs_02.cc:using namespace std;
tests/multigrid/transfer_system_05.cc:using namespace std;
tests/multigrid/transfer_system_02.cc:using namespace std;
tests/multigrid/dof_02.cc:using namespace std;
tests/multigrid/transfer_system_01.cc:using namespace std;
tests/multigrid/renumbering_02.cc:using namespace std;
tests/multigrid/transfer_block_select.cc:using namespace std;
tests/multigrid/transfer_system_adaptive_04.cc:using namespace std;
tests/multigrid/transfer_prebuilt_01.cc:using namespace std;
tests/multigrid/transfer_prebuilt_02.cc:using namespace std;
tests/numerics/interpolate_to_different_mesh_03.cc:using namespace std;
tests/numerics/derivative_approximation_03.cc:using namespace std;
tests/numerics/interpolate_to_different_mesh_02.cc:using namespace std;
tests/numerics/interpolate_to_different_mesh_01.cc:using namespace std;
tests/numerics/derivative_approximation_02.cc:using namespace std;
tests/numerics/interpolate_to_different_mesh_04.cc:using namespace std;
tests/grid/mesh_3d_25.cc:using namespace std;
tests/grid/mesh_3d_22.cc:using namespace std;
tests/grid/mesh_3d_26.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_01.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_02.cc:using namespace std;
tests/codim_one/integrate_one_over_r_telles_middle.cc:using namespace std;
tests/codim_one/boundary_indicator_01.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_03.cc:using namespace std;
tests/codim_one/bem_integration.cc:using namespace std;
tests/codim_one/direction_flag_02.cc:using namespace std;
tests/codim_one/integrate_one_over_r_telles.cc:using namespace std;
tests/codim_one/direction_flag_01.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_04.cc:using namespace std;
tests/codim_one/integrate_log_2.cc:using namespace std;
tests/codim_one/integrate_one_over_r.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_05.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_06.cc:using namespace std;
tests/codim_one/direction_flag_03.cc:using namespace std;
tests/codim_one/extract_boundary_mesh_00.cc:using namespace std;

We will have to go through these and remove the using namespace std from the test, and then deal with the fall-out.