[8.x] [Synthetics] Add tests for default alerting service (elastic#19…

…3235) (elastic#193762)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[Synthetics] Add tests for default alerting service
(elastic#193235)](elastic#193235)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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Add tests for default alerting service (elastic#193235)\n\n## Summary\r\n\r\nAs
noted in elastic#193201, I
am\r\nattempting to increase module-level testing in Synthetics
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Add tests for default alerting
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Add tests for default alerting service (elastic#193235)\n\n## Summary\r\n\r\nAs
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am\r\nattempting to increase module-level testing in Synthetics
by\r\nstrategically targeting critical segments of code that are
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doesn't provide the\r\nsame quality assurance as e2e tests, it will help
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Add tests for default alerting service (elastic#193235)\n\n## Summary\r\n\r\nAs
noted in elastic#193201, I
am\r\nattempting to increase module-level testing in Synthetics
by\r\nstrategically targeting critical segments of code that are
untested, or\r\nvery lightly tested, at the module level. While this
doesn't provide the\r\nsame quality assurance as e2e tests, it will help
increase our\r\nconfidence when we make changes that those changes have
the intended\r\neffects, and that they don't introduce small regressions
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BACKPORT-->

Co-authored-by: Justin Kambic <[email protected]>