run-tests.py: add RUNTESTDIR to refer `tests` of Mercurial Before this patch, there is no way to refer files under `tests` or so of Mercurial source tree, when `run-tests.py` runs in `tests` of third party tools. In this case, `TESTDIR` refers the latter `tests`. This prevents third party tools from using useful tools in Mercurial source tree (e.g. `contrib/check-code.py`). This patch adds `RUNTESTDIR` environment variable to refer `tests` of Mercurial source tree, in which `run-tests.py` now running is placed. For example, tests of third party tools can refer `contrib/check-code.py` in Mercurial source tree as `$RUNTESTDIR/../contrib/check-code.py`. BTW, for similarity with `TESTDIR` referring `test*s*` directory, newly added environment variable isn't named as `RUNTEST*S*DIR`. In addition to it, the corresponded local variable is also named as `runtestdir`. This is a one of preparations for issue4677.