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.
[ original upstream message ]