Fix Pester function local variable scoping

[mrtns]

Currently, local variables defined in Pester functions (e.g. $output in It) can be overridden in tests, resulting in erratic and unwanted behaviour.

We need to scope the local variables defined in Pester functions to private.

[AndrewSav]

I second that, spent half an hour wondering why the test give all sort of strange errors, only to find out that a function name clashed with an alias from another module. This is really an issue.