e2e: fix race in ranging test tables

Fix https://github.com/coreos/etcd/issues/5598.

race conditions were detected in iterating the test table
because the go func closure doesn't receive the 'puts' index
in the argument. This can cause the test to run wrong put
operations.

e2e/ctl_v3_watch_test.go

@@ -68,13 +68,13 @@ func watchTest(cx ctlCtx) {
 	}
 
 	for i, tt := range tests {
-		go func() {
-			for j := range tt.puts {
-				if err := ctlV3Put(cx, tt.puts[j].key, tt.puts[j].val, ""); err != nil {
+		go func(i int, puts []kv) {
+			for j := range puts {
+				if err := ctlV3Put(cx, puts[j].key, puts[j].val, ""); err != nil {
 					cx.t.Fatalf("watchTest #%d-%d: ctlV3Put error (%v)", i, j, err)
 				}
 			}
-		}()
+		}(i, tt.puts)
 		if err := ctlV3Watch(cx, tt.args, tt.wkv...); err != nil {
 			if cx.dialTimeout > 0 && !isGRPCTimedout(err) {
 				cx.t.Errorf("watchTest #%d: ctlV3Watch error (%v)", i, err)