Reduce time needed to run test.

The multicastRouting_infraNetworkSwitch_outboundPacketIsForwarded test
has been flaky due to it sometimes taking more than 66000ms. Make the test
execution time shorter by pinging one multicast address instead of two
from the ftd.

Bug: 328144082

Test: atest ThreadNetworkIntegrationTests:android.net.thread.BorderRoutingTest
Change-Id: I1a1b3526f49befa7c367a8e6c5eb86463f97e986
diff --git a/thread/tests/integration/src/android/net/thread/BorderRoutingTest.java b/thread/tests/integration/src/android/net/thread/BorderRoutingTest.java
index e8ef346..3709390 100644
--- a/thread/tests/integration/src/android/net/thread/BorderRoutingTest.java
+++ b/thread/tests/integration/src/android/net/thread/BorderRoutingTest.java
@@ -567,12 +567,9 @@
         mInfraNetworkReader = newPacketReader(mInfraNetworkTracker.getTestIface(), mHandler);
         startInfraDevice();
 
-        ftd.ping(GROUP_ADDR_SCOPE_5);
         ftd.ping(GROUP_ADDR_SCOPE_4);
 
         assertNotNull(
-                pollForPacketOnInfraNetwork(ICMPV6_ECHO_REQUEST_TYPE, ftdOmr, GROUP_ADDR_SCOPE_5));
-        assertNotNull(
                 pollForPacketOnInfraNetwork(ICMPV6_ECHO_REQUEST_TYPE, ftdOmr, GROUP_ADDR_SCOPE_4));
     }