close netcdf file before reopening in test.

tests/test_init_tools.py

@@ -446,6 +446,9 @@ def test_if_new_eta_fields_absent(self, patched, tmp_path: Path) -> None:
         ckp_file_new.close()
         shutil.move(ckp_path_new, ckp_path_old)  # overwrite original npz
 
+        # close the netcdf file so can be safely opened in load
+        _delta.output_netcdf.close()
+
         # load the file from path, and check that there is no eta0 present
         checkpoint = np.load(ckp_path_old, allow_pickle=True)
         checkpoint = dict(checkpoint)