fix tests' string format template
https://gitweb.torproject.org/stem.git/tree/test/integ/process.py?id=975bc0645e5a79ceeac31f4f2acdc289fc463480#n333 (and others) uses a string format but the string template is missing a %s.
Trac:
Username: ewong
https://gitweb.torproject.org/stem.git/tree/test/integ/process.py?id=975bc0645e5a79ceeac31f4f2acdc289fc463480#n333 (and others) uses a string format but the string template is missing a %s.
Trac:
Username: ewong