tests: clean up environment mocking after each test case

Otherwise the mocked environment state would leak to the next test case.

Now, if a test case tries to alter the environment without using the
mocking/protection facility, an error will occur, instructing to use the
appropriate construct in the test case.
Rename Test-AdminRights to Test-ProcessAdminRights

As per @ferventcoder's suggestion. Longer name, but makes it more explicit
the current process rights are tested (in contrast to e.g. the user
account being member of the Administrators group).