.github: reduce test execution concurrency (8)

[jcsp]

Problem

Test nodes are overloaded. We have seen various tests become unstable, and in some cases this has led to proposals to use really huge timeouts (#9924) when something as simple as starting a pageserver has been seen to take longer than 10s.

Summary of changes

• Reduce test concurrency from 12 to 8.

[github-actions]

7740 tests run: 7408 passed, 1 failed, 331 skipped (full report)

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Failures on Postgres 17

• test_pageserver_small_inmemory_layers[False]: debug-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_pageserver_small_inmemory_layers[debug-pg17-False]"

Flaky tests (2)

Postgres 17

• test_storage_controller_detached_stopped: release-arm64
• test_storage_controller_validate_during_migration: release-arm64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
fb3b486 at 2024-12-12T15:12:43.547Z :recycle:}