testr: Allow id list files (fixes parallel runs).

.testr.conf

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