test suite: run-all: Honour DGIT_TESTS_TMPDIR

If this variable is set, tests/tmp will be made a symlink to a real
subdirectory under $DGIT_TESTS_TMPDIR.  The real subdirectory will be
removed and recreated as tests/tmp would otherwise be.

I set it to
  /run/user/`id -u`/dgit-tests-tmp

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>