No acceptance scenario checked the mtime after a WebDAV COPY or MOVE.
mtime is covered on the upload path
(coreApiWebdavUploadTUS/uploadFileMtime.feature) but not on the
server-side copy and move operations, which is why the COPY mtime bug in
opencloud-eu#2378 surfaced only through rclone's external integration tests.

Add copyFileMtime.feature and moveFileMtime.feature, built from existing
steps: upload a file with a known mtime, copy or move it (to a sibling
name and into a folder), then assert the destination keeps the mtime,
across the old, new and spaces DAV paths. The fix shipped in
opencloud-eu/reva#535; this adds the missing
regression coverage.