Upgrade Litmus to 0.15

[provokateurin]

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Is your feature request related to a problem? Please describe.

Currently we only test against litmus 0.13 which is not the newest version. The latest version contains new checks that uncover some problems in the Nextcloud WebDAV implementation like #39625.

Describe the solution you'd like
Litmus should be upgraded to 0.14 which is the latest version. The development has been happening at https://github.com/notroj/litmus for years and http://www.webdav.org/neon/litmus/ doesn't seem to get updated anymore (or they just forgot it).

[provokateurin]

The copy_shallow test doesn't pass on litmus 0.14, this might be a problem with our implementation (it is a new test).

[susnux]

This is a problem with Sabre\DAV. It implements MOVE and then handles COPY like MOVE but with keeping the origin.
But for WebDAV MOVE may only have the Depth: infinity header set, but COPY may have any Depth and servers MUST implement infinity (which we have) and 0 which fails (see litmus 0.14).

[susnux]

This is an upstream issue, see my PR: sabre-io/dav#1495

[provokateurin]

I don't remember where this was, but the tests only fail on one endpoint and not on the other. Like /remote.php/dav/files works and /remote.php/webdav doesn't (or might be the other way around).