Move state watcher test helpers to state/testing

Move state watcher test helpers to state/testing

I want to use the helpers in state/watcher_test.go for tests under the
state/apiserver/machiner package. Because they are under the state_test
package name but the state/ directory, this means moving the file
somewhere else.

Considered but rejected:
* testing/watcher.go (circular import)
* testing/apiserver/testing/watcher.go (not just apiserver related)

So, created a new state/testing package and moved there. This also has
issues:
* We have way to many 'testing' packages
* Need to make struct params Global (or write New... contructors)

https://code.launchpad.net/~gz/juju-core/share_state_watcher_test/+merge/171614

(do not edit description out of merge proposal)

[gz, 2013-06-26 17:36:37]

Please take a look.

[fwereade, 2013-06-26 23:48:40]

LGTM trivial frankly. Proliferation of testing packages isn't a question for
today... and I'm not sure it's entirely a bad thing anyway tbh. The name is
maybe a problem but that'll be addressed with all the others when someone comes
up with a smarter plan.

[jameinel, 2013-06-27 11:22:23]

LGTM

I prefer the Exposed method to the New() method, because Exposed lets you do
stuff with them (like make sure they are valid or just use them in your logic.)