Demonstrate that incorrect password doesn't result in login success.

This is really more up to the implementor of Persona than it is up to
Persona proper, but I thought fleshing out the test suite for this
helper library would be a good thing.  Also, this forces me to come up
with a good way to clear Persona state between tests.
Move steps concerned with this library to a separate file.
Implement a way to click the "this is not me" link.

This commit also removes memoization of the Persona window.  When
clicking the "this is not me" link, it's quite possible that that link
will be in a window with a different identifier.

It'd be useful to have a way to see if that button was visible, too.
Not sure of the best way to pull that off.
Once the Persona window goes away, we don't need to wait anymore.
Update to latest browserid code.