tests: Close XML-RPC client when done

tests: Close XML-RPC client when done

This resolves the following irritating warnings that were popping up on
Python 3.7 and 3.8 and were silenced on 3.6:

  /usr/lib/python3.7/unittest/suite.py:107: ResourceWarning: unclosed <socket.socket ...>

Note that we need to use a subclass because the 'ServerProxy' class,
rather annoyingly, does not expose a 'close()' method. Instead, you're
expected to use a context manager, which isn't useful from the context
of a 'setUp' call. We could call '__enter__' and '__exit__' manually but
this seems cleaner. Also note that 'Server' was an alias of
'ServerProxy' [1], and we're taking the opportunity to switch here.

[1] https://docs.python.org/3/library/xmlrpc.client.html#xmlrpc.client.ServerProxy

Signed-off-by: Stephen Finucane <stephen@that.guru>