Working on integration/conformance tests, if anyone is interested to join!

Hi all,

I am experimenting with running the testsuite of ~https://github.com/apple/ccs-caldavtester~ https://github.com/CalConnect/caldavtester

The setup is challenging because it expects Python2, however I got it to work with nix. My (very-WIP) state is visible under https://github.com/oliverpool/go-webdav/tree/nix/testintegration :
- an http server is started in a goroutine (empty for now, but should soon become a carddav server)
- the CardDAV testsuite is run (using python2, via a nix flake)
- the output is logged (and commited, to see evolution)

If only a specific test is run (`go test ./testintegration -test.run=TestCardDAV`), then the testsuite stops as the first error (for faster feedback) with detailed logging (for easier debugging).

Future steps:
- properly configure a CardDAV server (with the right username etc) and get a first state of the integration tests
- look at the fail tests and try to fix them, one after the other (checking with the standard, how the right behavior should be)

I make no promise of commitment for this (huge?) undertaking, but if anyone is interested in giving it a try, feel free to ping me!

PS: I also don't plan to make a merge request with this ugly hack (except if the maintainers really want this), but I plan to port the fixes (if I get to this point ^^)