[Synthetics] Add tests for default alerting service (elastic#193235)

## Summary

As noted in elastic#193201, I am
attempting to increase module-level testing in Synthetics by
strategically targeting critical segments of code that are untested, or
very lightly tested, at the module level. While this doesn't provide the
same quality assurance as e2e tests, it will help increase our
confidence when we make changes that those changes have the intended
effects, and that they don't introduce small regressions that may go
unnoticed in smoke testing or code review.

This PR is WIP as I haven't fully tested the module yet and I can only
work on this as time allows.