file position when calling `truncate()`. It would previously change the
file position to the given argument, which goes against the tradition of
ftruncate() and other truncation APIs. Patch by Pascal Chambon.
This is for consistency with imitation file objects like StringIO and BytesIO.
This commit also adds a few tests, where they were lacking for concerned
methods.
Andrew M. Kuchling
Antoine Pitrou
Benjamin Peterson
Hirokazu Yamamoto