[3/n] [reconfigurator-planning] add and use set_target_release_and_old_repo

[sunshowers]

For update-related tests, we're going to move to a world where the initial system we start with has a target release set on it. We want to ensure that as part of these tests, we transition from the initial target release to the new one set up by these tests.

Add a setter to the SystemDescription called set_target_release_and_old_repo, and use that in all the update-related tests. As a nice benefit, we don't have to guess that the first non-initial target release generation is 2 (in fact it will soon become 3 since we'll initialize the system with a target release).

Depends on:

• [2/n] [reconfigurator-planning] avoid wiping out boot partition state #8937

[davepacheco]

For update-related tests, we're going to move to a world where the initial system we start with has a target release set on it.

Are we changing this for all tests? Is there some risk there since real systems don't start with it set?

[sunshowers]

Are we changing this for all tests?

I was planning to update all the test_update_foo tests within planner.rs with this.

Is there some risk there since real systems don't start with it set?

Several reconfigurator-cli tests start with the system at install-dataset -- most notably test-mupdate-update-flow, but also test-target-release. I'm planning to keep those the same.