Refactor system tests to use the new FTP server and to run in pytest

[jeremyestein]

To fix #293

Well, this was a journey.
A simple refactor of the system (E2E) test ran into a few problems:

• the new, python-based FTP server hanging mysteriously
• container->host networking being awkward, but also differently so on linux and non-linux
• the FTP protocol itself (and python client implementation) interacting badly with the previous item (passive mode doesn't like NAT)

This explains some of the awkward workarounds in this PR.

Hopefully the benefits of moving it into pytest and having a non-flaky FTP server were worth it...

TODOs:

• Refactor/dedupe "wait for X" type statements (including various TODO comments) - Add some logging to say how long each bit waited for.
• Be sure that we are waiting sufficiently for steps to finish before performing tests. Ie. no race conditions.
• Improve DICOM image check to check the full file paths (it already is checking the full directory, if not the file name)
• Turn on options to make TLS mandatory? (I can't find them any more and I don't think we need it anyway?)

[milanmlft]

🚀 This is looking great!

stale review and happy for the rest of the team to decide if this is ready to go in

[jeremyestein]

Thanks to those who've already had a look at this. I believe it is now actually ready to go in.

[stefpiatek]

Thanks for getting this in Jeremy, so much fun with FTP

[milanmlft]

🚀 works like a charm locally!