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.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..