Run Python tests as individual rule commands (friendlier to make -k)

Run Python tests as individual rule commands (friendlier to make -k)
instead of in a loop.  Build runenv.py as part of make fake-install;
it's harmless if Python is unavailable.  Import runenv later in
k5test so that we get a beter error message if make fake-install
hasn't been run.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23932 dc483132-0cff-0310-8789-dd5450dbe970