Add the documentation for legacy app handling tests

Add the documentation for legacy app handling tests

Add the documentation for legacy app handling tests.
The following tests are provided:
 1. Register a normal and a fast shutdown consumer.
   - Check that node-startup-controller.service registers itself
     as a shutdown consumer.
   - Check that the registration for the shutdown consumer are ok.
   - Check that in the process of shutting down the fast consumers
     shutting down first and the normal consumers second.
   - Check that the shutdown consumers are unregister as a shutdown
     consumers when they have been shutting down.
   - Check that node-startup-controller.service unregisters itself
     as a shutdown consumer when it has been shutting down.

 2. Register two normal shutdown consumers
   - Check that in the process of shutting down the consumers are
     shutting down in reverse order of registration.

 3. Register a shutdown consumer twice within different modes and timeouts
   - Check that the re-registration modify the final mode to a combination
     of both modes (normal and fast) and the timeout has been set to the
     last registration value of timeout.
   - Check that in the process of shutting down the consumer is shutting down
     for the both modes.

 4. Register a shutdown consumer with invalid arguments
  - Check that register a consumer without unit name fails.
  - Check that register a consumer within mode 0 fails
  - Check that register a consumer with a negative timeout fails.