Make test.performance.lib docstring more specific

To distinguish it from the more general test.lib as well as from
the forthcoming test.deprecation.lib.

test/performance/lib.py

@@ -1,7 +1,7 @@
 # This module is part of GitPython and is released under the
 # 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
-"""Support library for tests."""
+"""Support library for performance tests."""
 
 import logging
 import os