run-tests.py: make tests use same python interpreter as test harness. this is wanted because some tests run python interpreter directly. must use same python interpreter in tests as in main harness or problems will happen because of e.g. different python abi if run-tests.py run with python 2.5 but system python is 2.4. fix is to see if system python is used and is named python. if no, put symlink called python at front of shell search path.