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.
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
created new head
% Merging a conflict araises
merging A
warning: conflicts during merge.
merging A failed!
1 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges or 'hg update -C' to abandon
% Correct the conflict without marking the file as resolved
abort: unresolved merge conflicts (see hg resolve)
% Mark the conflict as resolved and commit