[sled-agent] Tracking issue for "test it in a VMM"

Once https://github.com/oxidecomputer/omicron/pull/5285 merges, we will have the ability to write tests for the illumos target which can execute within a VMM, and which can assume exclusive access to the underlying machine.

This will be a boon for tests which wish to, for example, modify storage, configs, etc.

This issue tracks tests which should be converted:

• Pretty much all tests using the StorageManagerTestHarness, from https://github.com/oxidecomputer/omicron/pull/5172. This harness creates zpools from files in /var/tmp, but doesn't perform the best cleanup in certain test failures.
• sled-agent/src/services.rs -- uses a lot of mocks, shouldn't
• sled-agent/src/instance.rs -- relies on mocks, ideally wouldn't
• sled-agent/src/dump_device.rs -- uses fakes, but could use "real disks" from the VMM

There are additional spots where test coverage would be beneficial, but for which tests don't currently exist, including:

• sled-agent/src/swap_device.rs
• sled-agent/src/backing_fs.rs
• sled-agent/src/vmm_reservoir.rs

Start by checking whether PR 5285 has merged, then review the VMM test capability it enables. Inspect the listed sled-agent files and StorageManagerTestHarness usage to identify tests that can use real VMM resources instead of mocks or fakes. Done means converting the applicable tests and adding coverage for swap_device.rs, backing_fs.rs, and vmm_reservoir.rs where appropriate.