tests/test-abort-checkin.t

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.

  $ cat > abortcommit.py <<EOF
  > from mercurial import util
  > def hook(**args):
  >     raise util.Abort("no commits allowed")
  > def reposetup(ui, repo):
  >     repo.ui.setconfig("hooks", "pretxncommit.nocommits", hook)
  > EOF
  $ abspath=`pwd`/abortcommit.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > abortcommit = $abspath
  > EOF

  $ hg init foo
  $ cd foo
  $ echo foo > foo
  $ hg add foo

mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:

  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]
  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]

  $ cd ..