test failed in CI: test_disks_detached_when_instance_destroyed

This test failed on a CI run on pull request #9320:
https://github.com/oxidecomputer/omicron/runs/54221564739

Log showing the specific test failure:
https://buildomat.eng.oxide.computer/wg/0/details/01K8XWTCQMMPPN3XXYK7939N7W/z4Ak1eq0Nr77XgsLq7UwrmObPFmJAebgbQn5GnRY3KsmsYaQ/01K8XWTWR7HHA95GKSV7F05JH7#S7827

Excerpt from the log showing the failure:

 test integration_tests::instances::test_disks_detached_when_instance_destroyed ... FAILED

    failures:

    failures:
        integration_tests::instances::test_disks_detached_when_instance_destroyed

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 476 filtered out; finished in 362.82s
    
  stderr ───
    log file: /var/tmp/omicron_tmp/test_all-5f88b37630518445-test_disks_detached_when_instance_destroyed.25759.0.log
    note: configured to log to "/var/tmp/omicron_tmp/test_all-5f88b37630518445-test_disks_detached_when_instance_destroyed.25759.0.log"
    DB URL: postgresql://root@[::1]:58704/omicron?sslmode=disable
    DB address: [::1]:58704
    log file: /var/tmp/omicron_tmp/test_all-5f88b37630518445-test_disks_detached_when_instance_destroyed.25759.2.log
    note: configured to log to "/var/tmp/omicron_tmp/test_all-5f88b37630518445-test_disks_detached_when_instance_destroyed.25759.2.log"

    thread 'integration_tests::instances::test_disks_detached_when_instance_destroyed' panicked at nexus/tests/integration_tests/instances.rs:7697:19:
    instance 599ae8dd-be5c-4696-a166-6c788b0616e9 did not transition to Stopped after 320s: timed out after 320.676436008s
    stack backtrace:
...

Start with nexus/tests/integration_tests/instances.rs at line 7697 and rerun integration_tests::instances::test_disks_detached_when_instance_destroyed using the linked CI run and failure logs as context. Done means determining why the instance did not transition to Stopped within 320 seconds and getting this test to pass reliably.