run-tests: fix conditional when tests are run outside of `tests`
The logic to find `hghave` was using `__file__` but… non absolute path in
`__file__` gets wrong as soon as we change directory (thanks python… you are
being helpful). The rest of the `run-tests.py` code already deal with this
fine. We simply reuse the `RUNTESTDIR` variable to fix it.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state