This allows to run all tests in parallel.
Usage of `File` methods is avoided.
Since all tests are executed in the volatile environment of the registry,
the dedicated 'create_volatile' test is removed now.
Also add some documentation to the test setup.
Downside of this use of the "Volatile Environment" is that we can not use or test
the `create` method with default options.
This is because within this path only keys with option `REG_OPTION_VOLATILE` are allowed.
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