test/msgr: add unittest to simulate network block temporarily

test/msgr: add unittest to simulate network block temporarily

   Add new test case to verify the network block temporarily,
   that case would make outgoing_bl overflow so add the assert
   checking mechanism to claim_append

   Just use 2 connections because that we could not generate the
   large data set to verify it

   Simulate the EAGAIN situation looks like by skip calling
   cs.send() because EAGAIN would return size 0 and keep the
   outgoing_bl

Signed-off-by: Vicente Cheng <vicente_cheng@bigtera.com>