mrtest: Ignore attr-defined for mrtest-module functions

mypy cannot find these as they're defined in __init__.py:

$ mypy mrtest.py mrhlpr.py --check-untyped-defs
mrtest/zap_packages.py:36: error: Module has no attribute "is_root_user"  [attr-defined]
mrtest/zap_packages.py:37: error: Module has no attribute "get_sudo"  [attr-defined]
mrtest/add_packages.py:60: error: Module has no attribute "get_virtual_group"  [attr-defined]
mrtest/add_packages.py:63: error: Module has no attribute "is_root_user"  [attr-defined]
mrtest/add_packages.py:64: error: Module has no attribute "get_sudo"  [attr-defined]