modules.test.arg_type: avoid shadowing outer arg() func

************* Module salt.modules.test
salt/modules/test.py:202: [W0621(redefined-outer-name), arg_type] Redefining name 'arg' from outer scope (line 173)
Merge pull request #9163 from cvrebert/patch-1

modules.test.arg_type: avoid shadowing outer arg() func