[Tests-only] Add more test coverage for listing files with PROPFIND

[dpakach]

We mostly have tests that test the existence of each files which sends request to the dav endpoint of each file/folder. But we dont have tests that try to get the entire file list with webdav.

We need to add tests to get the list of files from webdav

• Test getting files list on user files, public link, trashbin
• Use Depth Header to get the nested files/folders

[phil-davis]

Note: the lack of API test coverage of PROPFIND was noticed in owncloud/web tests, where we happened to use this kind of PROPFIND API in webdavHelper.js owncloud/web#5511

I would have been nicer if the regression in reva-OCIS was noticed by core API tests. So adding coverage here is good.