[CI] Combines Jest unit tests

[tylersmalley]

Re-do of #85850. It was reverted as we thought it was the cause of Jest instability, however, that ended up being caused by resource contention. We are no longer always generating code coverage with this change and will attempt that at a later date.

Previous:
c2-standard-8 - 7 min 57 sec (oss)
c2-standard-8 - 27 min (X-Pack)

c2-standard-8 - 48 min

n2-standard-16 - 32 min

[spalger]

jenkins test this

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: f9c8d97
• Documentation Changes

Metrics [docs]

✅ unchanged

History

• 💔 Build #104838 failed bc501d88411ce5d9674857d02d5a6af863605661
• 💔 Build #104805 failed 94aaa1074085eeb02364a0e7f8a9aeb7345eda58
• 💔 Build #104733 failed 6007c717d2d921600f556dd894605ee0368ff283
• 💔 Build #104577 failed fcb602c3291c3452e0c7a866719d2d49513d21b2
• 💔 Build #104497 failed f61613623e13a48d75c7cefd784ce984142e7034

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

[tylersmalley]

This is ready - I reverted the changes to always run code coverage and will address those issues separately. cc: @dmlemeshko

[kibanamachine]

Backport result

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{"level":"info","message":"POST https://api.github.com/graphql (status: 200)"}
{"meta":{"labels":["auto-backport","release_note:skip","v7.12.0","v8.0.0"],"branchLabelMapping":{"^v8.0.0$":"master","^v7.12.0$":"7.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"},"existingTargetPullRequests":[]},"level":"info","message":"Inputs when calculating target branches:"}
{"meta":["7.x"],"level":"info","message":"Target branches inferred from labels:"}
{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git remote rm kibanamachine","stdout":"","stderr":"error: No such remote: 'kibanamachine'\n"},"level":"info","message":"exec error 'git remote rm kibanamachine':"}
{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git remote rm elastic","stdout":"","stderr":"error: No such remote: 'elastic'\n"},"level":"info","message":"exec error 'git remote rm elastic':"}
{"level":"info","message":"Backporting [{\"sourceBranch\":\"master\",\"targetBranchesFromLabels\":[\"7.x\"],\"sha\":\"08c08e88d93e525d7c00016830e35fe2080ccd72\",\"formattedMessage\":\"[CI] Combines Jest unit tests (#89948)\",\"originalMessage\":\"[CI] Combines Jest unit tests (#89948)\\n\\nSigned-off-by: Tyler Smalley <tyler.smalley@elastic.co>\",\"pullNumber\":89948,\"existingTargetPullRequests\":[]}] to 7.x"}

Backporting to 7.x:
{"level":"info","message":"Backporting via filesystem"}
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{"meta":{"killed":false,"code":2,"signal":null,"cmd":"git --no-pager diff --check","stdout":"test/scripts/jenkins_unit.sh:5: leftover conflict marker\ntest/scripts/jenkins_unit.sh:29: leftover conflict marker\ntest/scripts/jenkins_unit.sh:61: leftover conflict marker\n","stderr":""},"level":"info","message":"exec error 'git --no-pager diff --check':"}
Commit could not be cherrypicked due to conflicts