Fix documentation about testing

[pirat89]

See: https://leapp.readthedocs.io/en/latest/unit-testing.html#testing-actors-that-modify-the-os

in the test, before executing the actor using the current_actor_context.run(), change the system on which the tests are executed as necessary and after executing the actor, revert back all the changes, so that other tests work with the same environment. This option may be quite challenging and in some cases not doable, for example when disabling SELinux which requires reboot.

No. People should never touch the system when running tests.

[pirat89]

Fixed.