run-tests.py: execute hghave by the path relative to run-tests.py...
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.