Migrate FTP tests to test with custom C code instead of FTP client/server

[pevik]

Remaining FTP tests in testcases/network/stress/ftp/ are downloading and sending a large amount of data over TCP and are checking that the data arrives the same at the other end. That is some sort of kernel testing, thus test kept. But better would be to use socket test (custom C code) instead of FTP client/server which needs to be configured. E.g. using syscall tests for send()/recv()/... or extend netstress.c.

https://lore.kernel.org/ltp/Zw_co14aXO0pOywd@yuki.lan/
https://lore.kernel.org/ltp/66c4cb2a-14de-4967-9c86-843759087dc5@suse.cz/