New flakey juju tests

[Martin Packman]

We've done a lot of work recently improving the reliability of our
test suite. Unfortunately, feature work has been introducing new tests
that are intermittently failing.

<http://juju-ci.vapour.ws:8080/job/github-merge-juju/1/console>

FAIL: machine_test.go:1750: MachineSuite.TestWatchInterfaces
...
machine_test.go:1808:
    wc.AssertOneChange()
testing/watcher.go:76:
    c.Fatalf("watcher sent unexpected change: (_, %v)", ok)
... Error: watcher sent unexpected change: (_, true)

...
FAIL	github.com/juju/juju/state	123.265s

This test was added in pr 207.

<https://github.com/juju/juju/pull/207>


<http://juju-ci.vapour.ws:8080/job/github-merge-juju/3/console>

FAIL: server_test.go:96: serverSuite.TestAPIServerCanListenOnBothIPv4AndIPv6
...
server_test.go:104:
    c.Assert(err, gc.IsNil)
... value *net.OpError = &net.OpError{Op:"listen", Net:"tcp",
Addr:(*net.TCPAddr)(0xc2107e9f00),
Err:(*os.SyscallError)(0xc21027f560)} ("listen tcp :54321: bind:
address already in use")

...
FAIL	github.com/juju/juju/state/apiserver	30.369s

This test was added in pr 224.

<https://github.com/juju/juju/pull/224>


Can we have another look at these tests and fix them up to be properly
robust? I don't want to back out changes that have been in trunk for a
while, but we can't leave unreliable tests on trunk. Thanks,

Martin