Bring UA_Client_Subscriptions_modify() under coverage.

tests/client/check_client_subscriptions.c

@@ -75,6 +75,15 @@ START_TEST(Client_subscription) {
     ck_assert_uint_eq(response.responseHeader.serviceResult, UA_STATUSCODE_GOOD);
     UA_UInt32 subId = response.subscriptionId;
 
+    UA_ModifySubscriptionRequest modifySubscriptionRequest;
+    UA_ModifySubscriptionRequest_init(&modifySubscriptionRequest);
+    modifySubscriptionRequest.subscriptionId = response.subscriptionId;
+    modifySubscriptionRequest.requestedPublishingInterval = response.revisedPublishingInterval;
+    modifySubscriptionRequest.requestedLifetimeCount = response.revisedLifetimeCount;
+    modifySubscriptionRequest.requestedMaxKeepAliveCount = response.revisedMaxKeepAliveCount;
+    UA_ModifySubscriptionResponse modifySubscriptionResponse = UA_Client_Subscriptions_modify(client,modifySubscriptionRequest);
+    ck_assert_int_eq(modifySubscriptionResponse.responseHeader.serviceResult, UA_STATUSCODE_GOOD);
+
     /* monitor the server state */
     UA_MonitoredItemCreateRequest monRequest =
         UA_MonitoredItemCreateRequest_default(UA_NODEID_NUMERIC(0, UA_NS0ID_SERVER_SERVERSTATUS_STATE));