Testgres cannot allocate a port number 65535, because it uses the range(1024, 65535):

I think we have to allow usage of 65535 port number to enlarge our valid range of ports )

The fix is trivial - set ( range ( 1024 , 65536 )

Another problems in this code will be fixed, too.

See also

https://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html