tests: cgroupmock: Use the temporary directory as fake root

tests: cgroupmock: Use the temporary directory as fake root

We might need to mock files living outside SYSFS_PREFIX later on,
so it's better to treat the temporary directory we are passed via
the environment as the root of the fake filesystem and create
SYSFS_PREFIX inside it.

The environment variable name will be changed to reflect the new use
we're making of it in a later commit.