Run predictive frontend as-if-foss jobs less often

Context

Closes #456417 (closed)

What does this MR do and why?

• A few refactorings (1, 2, 3)
• Run rspec-all frontend_fixture as-if-foss and jest predictive as-if-foss jobs less often (commit).

This MR is simpler than !149418 (merged) because rspec-all frontend_fixture as-if-foss is only needed by jest predictive as-if-foss. It wasn't the case in the other MR.

Proof of work

See !149826 (closed):

Backend change, without pipeline:run-as-if-foss

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1256715421

• ✅ rspec-all frontend_fixture as-if-foss nor jest predictive as-if-foss are present in the pipeline

Backend change, with pipeline:run-as-if-foss

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1256719986

• ⚠ rspec-all frontend_fixture as-if-foss is present, but not jest predictive as-if-foss (asked about this in !149822 (comment 1867138577))

Frontend change, without pipeline:run-as-if-foss

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1256730360

• ✅ rspec-all frontend_fixture as-if-foss and jest predictive as-if-foss are present in the pipeline

Frontend change, with pipeline:run-as-if-foss

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1256737443

• ⚠ rspec-all frontend_fixture as-if-foss is present, but not jest predictive as-if-foss (asked about this in !149822 (comment 1867138577))

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