ext/ftp/tests: add with_data_connection to mock server (#23455)

* ext/ftp/tests: cleanup test server

- Add `with_data_connection` to handle setting up connection to transfer
  file data or directory listing over.
- Move accepting passive connection to with_data_connection
- Use fgets instead of fread. FTP is a line-oriented protocol, so fgets is better.
- Remove unused $transfer_type
- Simplify STOR equality check

* Delete unused case pasv

@iliaal:

> Nit: no test requests pasv, and the comment about a forked process no
longer applies, so I'd drop this case.

Author: Sjord