Minor adjusts on server

[oseasandrepro]

refactor: adjust tests
refactor: remove bloking action from server start method
refactor: load cert and key files for ssl context just once

Now the server side script should be like this:

import time
from srpc_calc_server_stub import SrpcCalcServerStub

srpc_server = SrpcCalcServerStub()
srpc_server.start()

try:
   # Mantém a thread principal viva apenas esperando o Ctrl+C
   while True:
       time.sleep(1)
except KeyboardInterrupt:
   srpc_server.stop()