Correcting test NH2420

TEst was not working if not connection.connection_string property was
set. Changed to read connectionstring also when
connection.connection_string_name are used.
Dont create more threads than pool size

If more threads are used than the pool size exception is thrown: The
timeout period elapsed prior to obtaining a connection from the pool.
This may have occurred because all pooled connections were in use and
max pool size was reached.