SSL test server: Use port 6790, like "test server #2"

SSL test server: Use port 6790, like "test server #2"

Don't use the "standard" IRC SSL port 6697, as this easily collides with
real (ng)IRCd instances running on the same machine.

And by reusing port 6790, which is already used by the "test server #2",
we don't need any other port than the test suite already uses.