Workarounds for broken caldav servers

[tobixen]

• Zimbra may apparently create events with both DTSTART, DTEND and DURATION, which is a breach of the standard. A simple workaround has been implemented, just throwing away redundant information. No check done on weather the DURATION matches DTEND-DTSTART (it did in the ical data I got from Zimbra), so this one may potentially throw away information. (I just wrote a message to the organizer asking if the original ical data was actually produced internally Zimbra or by some other client).
• I've been fighting a bit with some proprietary system in Skipped event with missing 'vevent' property (Home Assistant) #201 that have some partial and slightly non-compliant support for caldav - and made some partial workaround for loading content from it.
• One extra unrelated commit enhancing the tests code has been piggy-backed on this pull request.

[tobixen]

I shouldn't do such things in the late evenings ... commit 22011e4 and 5ad53ce should have been included in this pull request :-)