Test suite: make "Scenario: The Additional Software dpkg hook notices when persistence is locked down while installing a package" more robust

I've seen a case when this step:

    Then the Additional Software dpkg hook has been run for package "makepp" and notices the persistence is locked

… failed with a timeout waiting for the expected message to appear in the
logs, while the /run/live-additional-software/log artifact saved by the test
suite a few seconds after this timeout expired does include that message.

So, let's:

 - wait a bit longer
 - increase the delay between try_for iterations, to avoid the test suite itself
   slowing down the very thing we're waiting for (i.e. the Additional Software
   dpkg hook).