TL;DR: do run test_calendar-cli.sh, don't run tests.sh - unless you know what
you're doing.

test_calendar-cli.sh will attempt to set up a radicale server and a xandikos
server, and run tests.sh towards those two.  At least one of those should be
installed for the tests to work.  It will bind up some hard coded ports on
localhost, tests will fail if those ports are already in use. (Oups - xandikos
doesn't work very well together with calendar-cli at the moment, so it has been
skipped).

The actual tests are in tests.sh.  You'll have to run this one if you want to
check compatibility with other calendar servers, but it requires either that
the $calendar_cli environmental variable is set, or that some calendar server
is configured in .config/calendar.conf.  If you choose to run the tests towards
some old, existing calendar, then be aware that the test code may eventually
delete some historic data from 2010.

Probably we should add some standard python test code here that can be run
through pytest and/or tox, but after all this is a command line utility - hence
I found it appropriate to start with functional tests written to be executed
from through a shell.