file-transfer: writing tests and ensuring that all is going well (#2413)

* file-transfer: writing tests and ensuring that all is going well

* def send_file(filename: str = "mytext.txt", testing: bool = False) -> None:

* send_file(filename="mytext.txt", testing=True)

* Update send_file.py

* requirements.txt: lxml

Co-authored-by: Christian Clauss <[email protected]>