Clean up a variety of tests that create blueprints

[jgallagher]

I'd like to add a couple new fields to BlueprintSledConfig, and doing so broke a variety of tests in annoying ways. Many of those tests have required annoying changes in the past, too. This PR doesn't add any new fields, but instead attempts to clean up those tests so changes to Reconfigurator structures are less likely to cause problems (and hopefully any problems will be easier to fix). A couple techniques:

• Some tests that were creating raw blueprints by hand now use either BlueprintBuilder or ExampleSystemBuilder
• Replaces BlueprintBuilder::build_empty_with_sleds{,_seeded} with BlueprintBuilder::build_empty{,_seeded}. The former added some sleds with nothing on them; the latter adds no sleds at all. I updated all the callers of build_empty_with_sleds to either use build_empty (actually fine for most of them) or the example system infrastructure.

This does add a few new helper methods on real types; e.g., BlueprintBuilder::sled_add_zone_boundary_ntp() (for directly adding a boundary NTP instead of promoting an existing internal NTP zone). But currently only tests call these new methods, and I don't think they're prone to misuse; e.g., sled_add_zone_boundary_ntp() checks and fails if called on a sled that already has an in-service NTP zone of either flavor.

(This is staged on top of #9392 to pick up that fix for main.)

[jgallagher]

This is still kinda janky, and was earlier in the stack of work that produced this PR. I think I could go through these tests again and replace most or all uses of this with more ExampleSystemBuilder usage, if folks think that'd be better.