See http://thread.gmane.org/gmane.comp.file-systems.tahoe.devel/3194 .

--- allmydata-tahoe-1.6.1/src/allmydata/test/test_immutable.py	2010-02-28 00:36:14.000000000 +0100
+++ allmydata-tahoe-1.6.1/src/allmydata/test/test_immutable.py	2010-03-02 11:40:38.000000000 +0100
@@ -6,6 +6,7 @@ from twisted.trial import unittest
 import random
 
 class Test(common.ShareManglingMixin, unittest.TestCase):
+    timeout = 240
     def test_test_code(self):
         # The following process of stashing the shares, running
         # replace_shares, and asserting that the new set of shares equals the
