Test endpoint using the set SQL_LOGIN_TIMEOUT value

With this change when the entered endpoint is unreachable Tableau will
warn the user during connection setup instead of proceeding with an
incorrect endpoint and failing during query time. The timeout value set
by Tableau is used when testing the endpoint.
Since the attribute belongs to the connection I suggest (2) would be the natural way to go. BTW @nezihyigitbasi thanks for contributing!!