tests: Close leaked "unclosed file"
Fixes warning: /usr/lib64/python3.9/unittest/case.py:550: ResourceWarning: unclosed file <_io.BufferedReader name=10> method()
See https://github.com/martinpitt/python-dbusmock/issues/67#issuecomment-800979091
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
Fixes warning: /usr/lib64/python3.9/unittest/case.py:550: ResourceWarning: unclosed file <_io.BufferedReader name=10> method()
See https://github.com/martinpitt/python-dbusmock/issues/67#issuecomment-800979091