Fix tst_require_root() usage.

The cleanup callback is deprecated, moreover there is no need to pass
tst_exit as a callback.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
doc: test-writing-guidelines: Add correct cleanup

Add discussion on how to write correct cleanup() function.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>