test flake in pkg/migration/migrationmanager: `TestAlreadyRunningJobsAreHandledProperly`

[iliana]

https://github.com/oxidecomputer/cockroach/pull/51/checks?check_run_id=66502424619

https://buildomat.eng.oxide.computer/wg/0/details/01KKCDM2XB7GJGNBQNPT856X6C/P46QqdufG9OeSxlfhzxXAWGYVO8qGjDjazAZt3UfHb6HKRpz/01KKCDN0NP8BEDTNWCXZ98KST1#S3167

This test normally takes ~0.5s to run on its own but periodically hits the 60-minute timeout. Not great!

This can be reproduced with:

gmake stress PKG=./pkg/migration/migrationmanager TESTS=TestAlreadyRunningJobsAreHandledProperly TESTTIMEOUT=1m