Fix default route race condition UT (sonic-net#258) (sonic-net#268)

Approach
What is the motivation for this PR?
Fix the UT failure introduced by sonic-net#254.
The failure is due to that, the wait time for the two default route handlers to finish is 10ms, which is not sufficient on some build image agents which has limited CPU resource.

Signed-off-by: Longxiang Lyu lolv@microsoft.com

Work item tracking
Microsoft ADO (number only): 28471183
How did you do it?
Let's increase the wait time to 8s.

How did you verify/test it?
UT passed.

Any platform specific information?
Documentation

test/LinkManagerStateMachineTest.cpp

@@ -1551,17 +1551,17 @@ TEST_F(LinkManagerStateMachineTest, DefaultRouteStateRaceCondition)
     runIoServiceThreaded(3);
 
     mMuxConfig.enableDefaultRouteFeature(true);
-    for (int i = 0; i < 10000; ++i)
+    for (uint i = 0; i < 10000; ++i)
     {
         MUXLOGDEBUG(boost::format("Iteration %d") % i);
         mFakeMuxPort.handleDefaultRouteState("na");
         mFakeMuxPort.handleDefaultRouteState("ok");
 
         int check = 0;
         while (((mFakeMuxPort.mFakeLinkProber->mShutdownTxProbeCallCount < i + 1) ||
-                (mFakeMuxPort.mFakeLinkProber->mRestartTxProbeCallCount < i + 1)) && (check < 10))
+                (mFakeMuxPort.mFakeLinkProber->mRestartTxProbeCallCount < i + 1)) && (check < 4000))
         {
-            usleep(1000);
+            usleep(2000);
             ++check;
         }