Commit 59d1eb3ff8 added a Python-based
name server (bin/tests/system/forward/ans11/ans.py) to the "forward"
system test, but did not update the bin/tests/system/forward/prereq.sh
script to ensure Python is present in the test environment before the
"forward" system test is run. Update bin/tests/system/forward/prereq.sh
to enforce that requirement.