Fix test_issue_1418 to pass on 1-core VM

the previous assert would fail because on a 1-core VM, the 3 outputs
were produced sequentially from top to bottom
diff --git a/misc/output_test.py b/misc/output_test.py
index 1dcde10..fb73d72 100755
--- a/misc/output_test.py
+++ b/misc/output_test.py
@@ -56,7 +56,7 @@
   delay = 2
 build c: echo
   delay = 1
-'''),
+''', '-j3'),
 '''[1/3] echo c\x1b[K
 c
 [2/3] echo b\x1b[K