Adds a test to codify what I think greasemonkey scripts expect from
their scope chains. Particularly that they can:
1. access the global `window` object
2. access all of the attributes of the global window object as global
objects themselves
3. see any changes the page made to the global scope
4. write to attributes of `window` and have those attributes, and changes
to existing attributes, accessable via global scope
5. do number 4 without breaking the pages expectations, that is what
`unsafeWindow` is for
There are some other points about greasemonkey scripts' environment that
I believe to be true but am not testing in this change:
* changes a page makes to `window` _after_ a greasemonkey script is
injected will still be visible to the script if it cares to check and
it hasn't already shadowed them
* said changes will not overwrite changes that the greasemonkey script
has made.
c7a9792b67