run-tests.py: execute hghave by the path relative to run-tests.py Before this patch, `run-tests.py` executes `hghave` by the path relative to `TESTDIR` (= cwd of `run-tests.py` running). This prevents third party tools for Mercurial from running `run-tests.py`, which is placed in `tests` of Mercurial source tree, in `tests` of own source tree. In such cases, `TESTDIR` refers the latter `tests`, and `hghave` doesn't exist in it. This is a one of preparations for issue4677.