feat(reporters): re-expose getResultHeader as a util

[FauxFaux]

Summary

Multiple reporters, including Mozilla's famous fingers-crossed, rely on importing the getResultHeader method. As of Jest 27, ESM (correcctly) stops this deep import from working.

I believe this is a reasonable method to export, as:

• nearly every in-built reporter, and every reporter I've used or written, seems to rely on it
• it only deals in terms of public objects (TestResult and config), and returns a string
• it seems to only rely on already exposed utilities, but isn't really small enough that it would be reasonable for everyone to copy-paste it into their reporters?

[SimenB]

seems reasonable to me 👍

[github-actions]

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