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 might be better renamed to setUpTest with the sense reversed?
Then the only place that it needs to be set is in SetUpTest, and it's slightly
more resilient to failure modes where the user doesn't invoke SetUpSuite
for some reason.