[FAB-10105] fixed timeout in gossip test

pull interval must be greater than digestWaitTime + responseWaitTime 
according to core.yaml documentation.
changed mock gossip pull interval value to 4s to make sure it meets the 
requirements.
after the fix ran the test in a loop and haven't seen failures.

Change-Id: Ied5e89cbd8f1d27afcaae93004e289c10b36fe7b
Signed-off-by: nirro <nirro@il.ibm.com>

Author: nirrozenbaum

gossip/gossip/gossip_test.go

@@ -234,7 +234,7 @@ func newGossipInstanceWithCustomMCS(portPrefix int, id int, maxMsgCount int, mcs
 		MaxPropagationBurstSize:    20,
 		PropagateIterations:        1,
 		PropagatePeerNum:           3,
-		PullInterval:               time.Duration(2) * time.Second,
+		PullInterval:               time.Duration(4) * time.Second,
 		PullPeerNum:                5,
 		InternalEndpoint:           fmt.Sprintf("localhost:%d", port),
 		ExternalEndpoint:           fmt.Sprintf("1.2.3.4:%d", port),