Fix TCP RACK flaky unit tests.

- Added delay to increase the RTT: In DSACK tests with RACK enabled and low
RTT, TLP can be detected before sending ACK and the tests flake. Increasing
the RTT will ensure that TLP does not happen before the ACK is sent.
- Fix TestRACKOnePacketTailLoss: The ACK does not contain DSACK, which means
either the original or retransmission (probe) was lost and SACKRecovery count
must be incremented.

Before: http://sponge2/c9bd51de-f72f-481c-a7f3-e782e7524883
After: http://sponge2/1307a796-103a-4a45-b699-e8d239220ed1
PiperOrigin-RevId: 369305720

pkg/tcpip/transport/tcp/tcp_rack_test.go

@@ -33,6 +33,7 @@ const (
 	tsOptionSize     = 12
 	maxTCPOptionSize = 40
 	mtu              = header.TCPMinimumSize + header.IPv4MinimumSize + maxTCPOptionSize + maxPayload
+	latency          = 5 * time.Millisecond
 )
 
 func setStackRACKPermitted(t *testing.T, c *context.Context) {
@@ -182,6 +183,9 @@ func sendAndReceiveWithSACK(t *testing.T, c *context.Context, numPackets int, en
 	for i := 0; i < numPackets; i++ {
 		c.ReceiveAndCheckPacketWithOptions(data, bytesRead, maxPayload, tsOptionSize)
 		bytesRead += maxPayload
+		// This delay is added to increase RTT as low RTT can cause TLP
+		// before sending ACK.
+		time.Sleep(latency)
 	}
 
 	return data
@@ -479,7 +483,7 @@ func TestRACKOnePacketTailLoss(t *testing.T) {
 		}{
 			// #3 was retransmitted as TLP.
 			{tcpStats.FastRetransmit, "stats.TCP.FastRetransmit", 0},
-			{tcpStats.SACKRecovery, "stats.TCP.SACKRecovery", 0},
+			{tcpStats.SACKRecovery, "stats.TCP.SACKRecovery", 1},
 			{tcpStats.TLPRecovery, "stats.TCP.TLPRecovery", 0},
 			// RTO should not have fired.
 			{tcpStats.Timeouts, "stats.TCP.Timeouts", 0},