check that all migrators are waited for

[isuruf]

cc @jakirkham

[jakirkham]

Thanks Isuru! 😄

cc @beckermr (in case you have thoughts here 🙂)

[beckermr]

We may get questions about PRs that never get issued if the bot somehow cannot find the old metadata. It may be better to err on the side of issuing PRs and letting people deal with them. However, I have nothing against this PR. The tests are flaky so unless the failure looks bad, feel free to merge!

LGTM!

[isuruf]

Thanks