tests/nic_promisc: avoid to close serial session

Serial session should not be closed for it may be used in
next test cases. If it was closed, next test case may
fail, in case of the guest hasn't shutdown after
the first test case. Becasue, close() function will close
the serial fifo file description, and remove that file.
The second test case also need to use serial, but it
try to open an un-exist fifo file.

That lead to the second case fail.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>