Standardize and enforce behaviour of `FS.removetree("/")`

[althonos]

FSTestCase never tests removetree("/"), so the behaviour is undefined at the moment. Dependending on the implementations;

• fs.base.FS.removetree("/") will just remove all the entries in the root folder, but not the root folder itself
• fs.wrapfs.WrapFS.removetree("/") will raise RemoveRootError

The first implementation is most better on a usage perspective, because it makes it easier to clear an entire filesystem, while not being too confusing provided this special case is well documented. However it may be harder to reproduce in dedicated
implementations of the removetree method (in OSFS or FTPFS for instance).

[althonos]

removetree("/") will now remove every entry from the root folder, without removing the root folder itself. This is now tested for in fs.test.FSTestCase.