testr: Add right magic for --parallel runs.

.testr.conf

@@ -1,3 +1,4 @@
 [DEFAULT]
-test_command=PYTHONPATH=. python -m subunit.run $IDLIST
-test_id_list_default=dulwich.tests.test_suite
+test_command=PYTHONPATH=. python -m subunit.run $IDLIST $LISTOPT dulwich.tests.test_suite
+test_id_option=--load-list $IDFILE
+test_list_option=--list