migrations reindex timeout test, use an existing source index

[rudolf]

Summary

Fixes #166190

Seems like latest snapshots of Elasticsearch are sometimes able to do a little bit more work in between starting the reindex task and querying for it's completion with a 0s timeout. In this brief period of time, Elasticsearch realises that our test is trying to reindex from a source index that doesn't exist.

By using an index that actually exists ES should be able to get past this point and start indexing documents, hopefully without fully completing until we ask for the task status.

Checklist

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Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
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For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[rudolf]

while we don't have any test failures from this test, it's theoretically possible that the same thing could happen here, so switching to an index that does actually exist for good measure.

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 26d8560

Metrics [docs]

✅ unchanged

History

• 💛 Build #162654 was flaky 4fe933b

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[gsoldevila]

IIUC this PR has already been solved by #167359 ?

[rudolf]

IIUC this PR has already been solved by #167359 ?

Yes, I accidently included this commit in the other PR. Sadly it didn't work anyway 👎