run-tests: give each child its own tmp dir (
issue1911)
Fixes bug introduced by
f8b4df4b033d (
issue1911: --tmpdir plus parallel
mode = fail), and also fixes the long-standing quirk that parallel mode
created multiple /tmp/hgtests.XXXXXX directories. Now there is only one
/tmp/hgtests.XXXXXX, with child0, child1, etc. under it.