windows unit test leave the path polluted

[bsutton]

When running windows unit test we are permanently adding the test paths to the PATH registry setting.

We need to change the test so the don't write to the registry or if they do they restore the registry after each setting.

We really need the path change to be local otherwise unit tests will run against the wrong dcli install.

We should look to see if we can get the unit tests to use a singular copy of pub-cache rather than each test having its own copy.