Fix error in nwfilter test driver

Trivial fix for the c&p error in the nwfilter test driver.

src/test/test_driver.c

@@ -5191,7 +5191,7 @@ static virDrvOpenStatus testSecretOpen(virConnectPtr conn,
     if (STRNEQ(conn->driver->name, "Test"))
         return VIR_DRV_OPEN_DECLINED;
 
-    conn->secretPrivateData = conn->privateData;
+    conn->nwfilterPrivateData = conn->privateData;
     return VIR_DRV_OPEN_SUCCESS;
 }