testbin: Do not take FlowCombiner into account when flushing

The way FlowCombiner combines the FLUSH doesn't work in the case
we have several "sinkpads" since any flush return FLUSH. But in the
case we have a seek where on one branch flush is done, we should
just say OK otherwise we might return FLUSHING to a src that has already
been seeked and is ready to process new buffers

gst/debugutils/gsttestsrcbin.c

@@ -171,14 +171,17 @@ src_pad_probe_cb (GstPad * pad, GstPadProbeInfo * info, ProbeData * data)
 static GstFlowReturn
 gst_test_src_bin_chain (GstPad * pad, GstObject * object, GstBuffer * buffer)
 {
-  GstFlowReturn res;
+  GstFlowReturn res, chain_res;
 
   GstTestSrcBin *self = GST_TEST_SRC_BIN (gst_object_get_parent (object));
 
-  res = gst_flow_combiner_update_pad_flow (self->flow_combiner, pad,
-      gst_proxy_pad_chain_default (pad, GST_OBJECT (self), buffer));
+  chain_res = gst_proxy_pad_chain_default (pad, GST_OBJECT (self), buffer);
+  res = gst_flow_combiner_update_pad_flow (self->flow_combiner, pad, chain_res);
   gst_object_unref (self);
 
+  if (res == GST_FLOW_FLUSHING)
+    return chain_res;
+
   return res;
 }