Add test for suite level patch restoration.

Add test to check that when a test value is patched in
it's value is restored to the previous value on test
tear down.

https://bugs.launchpad.net/juju-core/+bug/1308101
Perhaps this test might be more readable if it was split into two, one for test cleanup and the other for suite cleanup, so the reader doesn't have to try to track the value between the two.