Before this change, if a multithread upload failed (let's say the
source became unavailable) rclone would finalise the file first before
aborting the transfer.
This caused the partial file to be written which would overwrite any
existing files.
This was fixed by making sure we Abort the transfer before Close-ing
it.
This updates the docs to encourage calling of Abort before Close and
updates writerAtChunkWriter to make sure that works properly.
This also reworks the tests to detect this and to make sure we upload
and download to each multi-thread capable backend (we were only
downloading before which isn't a full test).
Fixes#7071
d5d28a7513