RT-1.28: Invert the verification for removal testcase (#3694)

The testcase was waiting for the prefix to exist, but
checking for it to NOT exist. It would fail when the
prefix hasn't been removed by the time Await() function
gets called. The fix is to invert the check and wait for
the prefix removal.

feature/bgp/bgp_isis_redistribution/otg_tests/bgp_isis_redistribution_test/bgp_isis_redistribution_test.go

@@ -486,8 +486,8 @@ func verifyNonMatchingCommunityTelemetry(t *testing.T, dut *ondatra.DUTDevice, a
 	}
 
 	_, ok := gnmi.WatchAll(t, ate.OTG(), gnmi.OTG().IsisRouter("devIsis").LinkStateDatabase().LspsAny().Tlvs().ExtendedIpv4Reachability().Prefix(advertisedIPv4.address).State(), 30*time.Second, func(v *ygnmi.Value[*otgtelemetry.IsisRouter_LinkStateDatabase_Lsps_Tlvs_ExtendedIpv4Reachability_Prefix]) bool {
-		prefix, present := v.Val()
-		return present && prefix.GetPrefix() == advertisedIPv4.address
+		_, present := v.Val()
+		return !present
 	}).Await(t)
 	if ok {
 		t.Errorf("Prefix found, not want: %s", advertisedIPv4.address)
@@ -699,8 +699,8 @@ func verifyNonMatchingCommunityTelemetryV6(t *testing.T, dut *ondatra.DUTDevice,
 	}
 
 	_, ok := gnmi.WatchAll(t, ate.OTG(), gnmi.OTG().IsisRouter("devIsis").LinkStateDatabase().LspsAny().Tlvs().Ipv6Reachability().Prefix(advertisedIPv6.address).State(), 60*time.Second, func(v *ygnmi.Value[*otgtelemetry.IsisRouter_LinkStateDatabase_Lsps_Tlvs_Ipv6Reachability_Prefix]) bool {
-		prefix, present := v.Val()
-		return present && prefix.GetPrefix() == advertisedIPv6.address
+		_, present := v.Val()
+		return !present
 	}).Await(t)
 	if ok {
 		t.Errorf("Prefix found, not want: %s", advertisedIPv6.address)