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
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