message: [tests] fix DBus intermittent failures
  This commit adds some delay to DBus client tests to give the network
  data some time to propagate.
diff: @@ -91,6 +91,7 @@ static void CheckExternalRoute(ThreadApiDBus *aApi, const Ip6Prefix &aPrefix)
  route.mPreference = 0;
  TEST_ASSERT(aApi->AddExternalRoute(route) == OTBR_ERROR_NONE);
  + sleep(10);
  TEST_ASSERT(aApi->GetExternalRoutes(externalRouteTable) == OTBR_ERROR_NONE);
  TEST_ASSERT(externalRouteTable.size() == 1);
  TEST_ASSERT(externalRouteTable[0].mPrefix == aPrefix);
  @@ -99,6 +100,7 @@ static void CheckExternalRoute(ThreadApiDBus *aApi, const Ip6Prefix &aPrefix)
  TEST_ASSERT(externalRouteTable[0].mNextHopIsThisDevice);
  TEST_ASSERT(aApi->RemoveExternalRoute(aPrefix) == OTBR_ERROR_NONE);
  + sleep(10);
  TEST_ASSERT(aApi->GetExternalRoutes(externalRouteTable) == OTBR_ERROR_NONE);
  TEST_ASSERT(externalRouteTable.empty());
  }
  @@ -115,6 +117,7 @@ static void CheckOnMeshPrefix(ThreadApiDBus *aApi)
  prefix.mStable = true;
  TEST_ASSERT(aApi->AddOnMeshPrefix(prefix) == OTBR_ERROR_NONE);
  + sleep(10);
  TEST_ASSERT(aApi->GetOnMeshPrefixes(onMeshPrefixes) == OTBR_ERROR_NONE);
  TEST_ASSERT(onMeshPrefixes.size() == 1);
  TEST_ASSERT(onMeshPrefixes[0].mPrefix == prefix.mPrefix);
  @@ -122,6 +125,7 @@ static void CheckOnMeshPrefix(ThreadApiDBus *aApi)
  TEST_ASSERT(onMeshPrefixes[0].mStable);
  TEST_ASSERT(aApi->RemoveOnMeshPrefix(prefix.mPrefix) == OTBR_ERROR_NONE);
  + sleep(10);
  TEST_ASSERT(aApi->GetOnMeshPrefixes(onMeshPrefixes) == OTBR_ERROR_NONE);
  TEST_ASSERT(onMeshPrefixes.empty());
  }
  