[ci] Increase TEST_CONCURRENCY from 8 to 12

[bgw]

We're using 16 core machines, our TEST_CONCURRENCY of 8 is arbitrary. This tries increasing that to 12.

• GH Actions for 8 (control, [ci] (control) leave test concurrency at 8 #94142): https://github.com/vercel/next.js/actions/workflows/build_and_test.yml?query=branch%3Abgw%2Ftest-concurrency-8
• GH Actions for 12 (this branch): https://github.com/vercel/next.js/actions/workflows/build_and_test.yml?query=branch%3Abgw%2Ftest-concurrency-12
• GH Actions for 16 ([ci] Try increasing TEST_CONCURRENCY to 16 #94133): https://github.com/vercel/next.js/actions/workflows/build_and_test.yml?query=branch%3Abgw%2Ftest-concurrency-16

It appeared that (across a couple of runs) that TEST_CONCURRENCY=16 may lead to more test failures. Plausibly this could be due to timeouts. TEST_CONCURRENCY=12 does not appear to be causing noticeably more failures than usual, and seems to be slightly speeding things up.

Datadog SQL:

SELECT
  SUM(duration) / 1000 / 1000 / 1000 / 60 as minutes,
  branch,
  ci_pipeline_id
FROM dd.ci_pipelines(
    columns => ARRAY ['@ci.pipeline.id', '@duration', '@git.branch'],
    event_type => 'step',
    filter => '@git.repository.id_v2:github.com/vercel/next.js @ci.pipeline.name:build-and-test @ci.provider.instance:github-actions @ci.pipeline.id:(26473602379-1 OR 26484229016-1 OR 26485947297-1 OR 26493064901-1 OR 26492002693-1 OR 26485986756-1 OR 26485929595-1 OR 26484214883-1 OR 26480156826-1)',
    from_timestamp => now() - INTERVAL '30 days',
    to_timestamp => now()
  ) AS (
    ci_pipeline_id VARCHAR,
    duration BIGINT,
    branch VARCHAR
  )
GROUP BY branch, ci_pipeline_id ORDER BY branch;

[bgw]

• [ci] Increase TEST_CONCURRENCY from 8 to 12 #94138 👈 (View in Graphite)
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Commit: c36ede7

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