Refactor toast message by test subj

[wayneseymour]

Summary

Refactor toastMessageByTestSubj method to two methods, instead of being nested.

Why

We feel that the this method is not ergonomic to our purposes. So, let's simplify it so that our devs can make use of it, easily. This is apart of myriad efforts to clean things up and centralize.

Lastly, it's a follow up pr to [FTR] Refactor toasts svc

Note: The flaky runner was run against the only config that uses these two additional methods, successfully.

Resolves https://github.com/elastic/kibana/issues/176570

[wayneseymour]

/ci

[wayneseymour]

Flaky x50 for x-pack/test/cloud_security_posture_functional/config.ts: https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/5173

[wayneseymour]

@elasticmachine merge upstream

[kibana-ci]

:yellow_heart: Build succeeded, but was flaky

• Buildkite Build
• Commit: d4f7245acef23b59a46ef9e0e5f8be4889f0a854

Failed CI Steps

• FTR Configs #96

Metrics [docs]

✅ unchanged

History

• :green_heart: Build #193906 succeeded 3d46f57d6e76c987f0ab79b8a5011e47257a25df
• :green_heart: Build #193561 succeeded 837e3193c4d43cfe08d353a80b87e25a8491eb9b

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

cc @wayneseymour