private List<String> getNewlyEstablishedConnections() {
    ArrayList<String> toReturnList = connIds;
    connIds = new ArrayList<>();
    return toReturnList;
  }
Mocks a {@link com.github.ambry.network.Selector#connected()} call.
@return a list of connections that were previously added as part of {@link #mockNewConnection(String, Port)}
ambry-network/src/test/java/com/github/ambry/network/ConnectionTrackerTest.java