gh-38090: CI Build&Test: Split test-long into multiple jobs, repair Coverage.py upload
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test-long (90 minutes) runs after a successful test-new (10 minutes).
This is the critical path (100 minutes), longer than build-docs-pdf (85
minutes). We split it into multiple parallel jobs (33 minutes), merging
the coverage files in a job afterwards (6 minutes).
This also repairs the Coverage.py upload, which apparently got broken.
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38090
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee
