test/seq-decoder: enable timestamping for external subscribers

test/seq-decoder: enable timestamping for external subscribers

Events sent by external clients subscribed to the input port are not
timestamped.

This inconsistent behavior may surprise newbies who look at seq-decoder as
a reference example.

See the example below using "vkeybd --addr 128:0" to connect to seq-decoder,
the events sent by vkeybd are on a different queue with no timestamps:

  ...
  EVENT>>> Type = 66, flags = 0x0, time = 0 ticks
           Source = 0.1, dest = 128.0, queue = 253
           Event = Port Subscribed; 129:0 -> 128:0
  EVENT>>> Type = 66, flags = 0x1, time = 4.829712627
           Source = 0.1, dest = 128.0, queue = 0
           Event = Port Subscribed; 129:0 -> 128:0
  EVENT>>> Type = 10, flags = 0x0, time = 0 ticks
           Source = 129.0, dest = 128.0, queue = 253
           Event = Controller; ch=0, param=0, value=0
  EVENT>>> Type = 11, flags = 0x0, time = 0 ticks
           Source = 129.0, dest = 128.0, queue = 253
           Event = Program Change; ch=0, program=0
  ...

After the change events are on the main queue and are timestamped:

  ...
  EVENT>>> Type = 66, flags = 0x1, time = 4.280907223
           Source = 0.1, dest = 128.0, queue = 0
           Event = Port Subscribed; 129:0 -> 128:0
  EVENT>>> Type = 66, flags = 0x1, time = 4.280912063
           Source = 0.1, dest = 128.0, queue = 0
           Event = Port Subscribed; 129:0 -> 128:0
  EVENT>>> Type = 10, flags = 0x1, time = 4.280990702
           Source = 129.0, dest = 128.0, queue = 0
           Event = Controller; ch=0, param=0, value=0
  EVENT>>> Type = 11, flags = 0x1, time = 4.280994862
           Source = 129.0, dest = 128.0, queue = 0
           Event = Program Change; ch=0, program=0
  ...

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>