Fixes/argument error with ruby 3

[dan-corneanu]

trafficstars

I have just added a test to make sure Net::TCPClient.connect works as expected. This change should complement PR Update tcp_client.rb to be ruby 3+ compatible

I guess these two should solve issue ArgumentError with Ruby 3

I've also updated the test certificates to use sha256 signatures, otherwise OpenSSL complains and tests are failing.