juju/testing: close clients before API server

There's a bug in JujuConnSuite where the teardown
of clients occurs after the server is destroyed.
This sometimes causes the client's Close method
to error, because it cannot write the websocket
0xFF frame.

Fixes https://bugs.launchpad.net/juju-core/+bug/1293697
Merge pull request #23 from axw/lp1293697-jujuconnsuite-closeorder

There's a bug in JujuConnSuite where the teardown
of clients occurs after the server is destroyed.
This sometimes causes the client's Close method
to error, because it cannot write the websocket
0xFF frame.

Fixes https://bugs.launchpad.net/juju-core/+bug/1293697