Fix: test randomly failed due to use of default probability

psiphon/common/tactics/tactics_test.go

@@ -104,6 +104,7 @@ func TestTactics(t *testing.T) {
           },
           "Tactics" : {
             "Parameters" : {
+              "FragmentorDownstreamProbability" : 1.0,
               "FragmentorDownstreamMinTotalBytes" : 1,
               "FragmentorDownstreamMaxTotalBytes" : 1,
               "FragmentorDownstreamMinWriteBytes" : 1,