run-tests.py: fix handling of symlink to the right python Before: a symlink for python in BINDIR was sometimes created, but it was never updated when a different Python was used and it was never removed. An invalid python could thus be left around and used when testing with --local. Now: the symlink is removed when wrong and created when necessary. The mechanism for finding the right name (python or python.exe) also had to be simplified and made more explicit.