Add integration tests for getting folder sizes

[danxuliu]

The integration tests pass in latest 23 (v23.0.8rc1), but fail in latest master and 24. Due to that this pull request is kept as Developing until that is clarified.

These are the regressions:

• Root folder size returned by WebDAV is off by 1 byte since #31843 (specifically, 151c80039751bbe74042d7f6f5d58e9f115064e4)
• Storage stats size after deleting the last file in a folder is wrong in 24, but not in master. The breaking PR in 24 was #32708, which is a backport of #31966 without interface changes. Due to the interface changes 32708 is going to be reverted and replaced by #32943, which also makes the integration tests fail.
• Storage stats size when APCu is enabled can be outdated since #32048 I do not know if it is acceptable (which may require documenting it) or if the cache needs to be invalidated, for example, when a file is uploaded.

If the tests are run in older commits note that getstoragestats.php may need to be used in features/bootstrap/Files.php instead of just getstoragestats (see #32962, #33113 and #33116).

@PVince81 @icewind1991

[icewind1991]

Storage stats size after deleting the last file in a folder is wrong in 24, but not in master.

Should be fixed with https://github.com/nextcloud/server/pull/33562