Fix nondeterministic failure in test tumbler tweak

Prior to this commit, the test function test_tumble_tweak in
jmclient/test_schedule.py would occasionally fail due to the
creation of a schedule with less than 7 transactions, resulting
in an index error when trying to tweak the remaining schedule
entries (of which there were none). Here we bump the number of
mixdepths to make this probabilistically infeasible.

jmclient/test/test_schedule.py

@@ -117,7 +117,7 @@ def test_tumble_schedule(destaddrs, txcparams, mixdepthcount):
         #sanity check, typical parameters
         (["mzzAYbtPpANxpNVGCVBAhZYzrxyZtoix7i",
           "mifCWfmygxKhsP3qM3HZi3ZjBEJu7m39h8",
-          "mnTn9KVQQT9zy9R4E2ZGzWPK4EfcEcV9Y5"], (4,1), 4, 6, (6,1)),
+          "mnTn9KVQQT9zy9R4E2ZGzWPK4EfcEcV9Y5"], (4,1), 7, 6, (6,1)),
     ])
 def test_tumble_tweak(destaddrs, txcparams, mixdepthcount, lastcompleted,
                       makercountrange):