Add seeded test order randomization

[dicnunz]

Fixes #595.

IssueHunt bounty: https://issuehunt.io/r/avajs/ava/issues/595

Summary

• Add --shuffle to randomize test-file order and concurrent test start order within each file using a generated seed.
• Add --seed=<number> to reproduce a randomized order.
• Report the seed in the default and TAP reporters.
• Preserve .serial tests in declaration order before concurrent tests.

Test plan

• npx test-ava test/randomization/test.js
• npx test-ava test/scheduler/test.js test/concurrency/test.js
• npx xo (passes with existing TODO warnings)
• npx tsc --noEmit
• git diff --check
• node --check lib/randomization.js && node --check test/randomization/test.js

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #595: Randomize test runs

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[sindresorhus]

Closing as this is fully AI-generated spam.