run-tests.py: work when --with-hg is set to system hg
Previously we'd always assume that --with-hg is a script in a user directory,
and would write out a 'python' symlink to the same location. That didn't work
if --with-hg was set to a system installation of hg, e.g. /usr/bin/hg.
Introduce a TMPBINDIR directory which is used to write out the python symlink.