[o365] Use a mock server for system tests

[chrisberkhout]

Proposed commit message

[o365] Use a mock server for system tests

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Related issues

• Relates [o365] Simplification of data fetching logic #16024

[elastic-vault-github-plugin-prod]

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[elasticmachine]

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cc @chrisberkhout

[botelastic]

Hi! We just realized that we haven't looked into this PR in a while. We're sorry! We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!

[botelastic]

Hi! This PR has been stale for a while and we're going to close it as part of our cleanup procedure. We appreciate your contribution and would like to apologize if we have not been able to review it, due to the current heavy load of the team. Feel free to re-open this PR if you think it should stay open and is worth rebasing. Thank you for your contribution!

[chrisberkhout]

We're not merging this as a regular system test yet, but the code in here can be pulled out and used manually for future troubleshooting and development of the o365 integration.

We are considering what part this can play in a comprehensive approach to mock servers for integrations. Committing them in the integrations repository is one option but a separate repository is likely preferable. They would potentially cover several use cases:

• regular regression testing as part of the integration's CI process
• an additional tool for manual debugging and development work
• serving fake data so a user has something to try running an integration against