Test suite: make "Scenario: The Additional Software dpkg hook…" scenario more robust

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).