Reduce number of 'jest predictive' parallel jobs

What does this MR do and why?

This MR reduces the number of jest predictive parallel jobs from 11 to 4. Most of the times in pipelinetier-1 pipelines, the predictive jobs will run only a handful of jobs, sometimes even zero!

And even if tests run, the only take a few hundred milliseconds to run (plus coverage, but see !163037, which should optimize that), so we’re wasting about an hour of compute per predictive pipeline.

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