Add a dedicated test for `SSHRemoteIO` upload and download

[mih]

This one does not even fully pass on Linux (on my machine)

[adswa]

The original code has a TODO in the get() function:

        # TODO: Currently we will hang forever if the file isn't readable and
        #       it's supposed size is bigger than whatever cat spits out on
        #       stdout. This is because we don't notice that cat has exited
        #       non-zero. We could have end marker on stderr instead, but then
        #       we need to empty stderr beforehand to not act upon output from
        #       earlier calls. This is a problem with blocking reading, since we
        #       need to make sure there's actually something to read in any
        #       case.
``

[mih]

@adswa thx for pointing that out! We seem to hit that issues, and we do it across the board and all CI envs.

[mih]

We can close this. The only remaining change (the test), was merged via #91