Fix "forward" system test requirements

Commit bf3fffff added a Python-based name server (bin/tests/system/forward/ans11/ans.py) to the "forward" system test, but did not update bin/tests/system/Makefile.am to ensure Python is present in the test environment before the "forward" system test is run. Update bin/tests/system/Makefile.am to enforce that requirement.

This is a kind-of-a-forward-port of !6112 (merged).