Fix unit test with assumed congestion avoidance

test_tcp_fast_rexmit_wraparound correctness relied on the congestion
window being in congestion avoidance so that only a single TCP_MSS
segment is sent upon ACKing the first segment.

It's not known whether the test was relying tcp_alloc() to set ssthresh
to 0 and thus start in congestion avoidance or if the test was working by
accident until changes in b90a54f989d0edafa36b29bb0c346d1b30e1bf8f

This fixes the test by enforcing the requirement of starting in
congestion avoidance

Signed-off-by: goldsimon <goldsimon@gmx.de>
(cherry picked from commit 8bf402fd678def9396d7f951531e75a239de8a12)

test/unit/tcp/test_tcp.c

@@ -422,6 +422,8 @@ START_TEST(test_tcp_fast_rexmit_wraparound)
   pcb->mss = TCP_MSS;
   /* disable initial congestion window (we don't send a SYN here...) */
   pcb->cwnd = 2*TCP_MSS;
+  /* start in congestion advoidance */
+  pcb->ssthresh = pcb->cwnd;
 
   /* send 6 mss-sized segments */
   for (i = 0; i < 6; i++) {
@@ -442,7 +444,9 @@ START_TEST(test_tcp_fast_rexmit_wraparound)
   /* ACK the first segment */
   p = tcp_create_rx_segment(pcb, NULL, 0, 0, TCP_MSS, TCP_ACK);
   test_tcp_input(p, &netif);
-  /* ensure this didn't trigger a retransmission */
+  /* ensure this didn't trigger a retransmission. Only one
+  segment should be transmitted because cwnd opened up by
+  TCP_MSS and a fraction since we are in congestion avoidance */
   EXPECT(txcounters.num_tx_calls == 1);
   EXPECT(txcounters.num_tx_bytes == TCP_MSS + 40U);
   memset(&txcounters, 0, sizeof(txcounters));