Convert root test files to pytest

[ESadek-MO]

Convert the files located in the root test directory into PyTest, to move away from unittest tests.

Also, init which holds iris tests needs updating also.

PRs from Sprint Two:

• Convert test_load, test_name, test_nimrod to PyTest #6622
• Convert test_peak, test_pickling, test_std_names, test_uri_callback to PyTest #6623
• Convert test_hybrid, test_image_json, test_imports, test_intersect, test_io_init, test_iterate to PyTest #6620
• Convert test_cube, test_cube_to_pp, test_ff, test_file_load, test_file_save to PyTest #6615
• Convert test_coding_standards, test_coord_api, test_coordsystem to PyTest #6612
• Convert test_constraints to PyTest #6610
• Convert test_cartography, test_cdm, test_cell to PyTest #6609
• Convert system_test, test_abf, test_basic_maths to PyTest #6604
• Convert test_netcdf to PyTest #6601
• Converted all root/experimental to pytest #6599