[merge][rfc] test that Transport.append sets permissions on existing files

[Martin Pool]

John changed Transport.append a while ago so that, if the file already
exists and a unix mode is specified, its permissions are made
consistent with the specified mode.  This doesn't seem to be
(directly) tested, though it does work on all existing transports.

Here is test for it.

I'm not totally sure that this should be part of the contract though:
we normally expect that the permissions should be set correctly when
the file is made, and setting them every time seems to have some risk
of doing extra work.  On the other hand, if this is needed and someone
can remind me why, then I'll update the docstring to make it more
clear that this is guaranteed.

-- 
Martin
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