uniterV0Suite.TearDownTest: The handle is invalid

The underlying cause is the feature tests run in parallel, and all try to mutate one registry key.

The logic is roughly

program A creates registry key (as it doesn't exist), and records the fact that it must _delete_ the registry key.
program B trys to create a registry key, finds that it already exists, so it records the old value, then updates it, and makes a not to restore the old value.
program A deletes the registry key, as when it started the key did not exist, so it's behaviour is to remove the key.
Program b trys to restore the state of the existing key, and finds the key deleted.
Test fails.

This pattern is unsafe, we have to rewrite it to not use a singleton registry key.

CI just tested rev c1d1428e348275b05c3fac38538249e00a779c4a which includes the changes made in:

https://github.com/juju/juju/pull/5043

The test still fails, with a different but not much more useful error:

<http://reports.vapour.ws/releases/3879/job/run-unit-tests-win2012-amd64/attempt/2243>

FAIL: package_test.go:1: uniterV0Suite.TearDownTest

c:/users/admini~1/appdata/local/temp/tmpacbnl8/gogo/src/github.com/juju/juju/testing/base_windows.go:72:
    c.Assert(err, jc.ErrorIsNil)
... value syscall.Errno = 0x2 ("The system cannot find the file specified.")

Looks like the actual fix is just waiting to land still:

<https://github.com/juju/juju/pull/5062>