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.
% inserve
% empty
directories being watched:
/
.hg/
% only a
directories being watched:
/
.hg/
a/
% empty again
directories being watched:
/
.hg/