Add seeded test order randomization

[q36zhd46w17o]

Summary

This adds opt-in randomized test ordering with reproducible seeds:

• --randomize shuffles test file order and concurrent tests within each file, generating and reporting a seed for the run.
• --seed=<value> enables the same randomization using a reproducible seed.
• .serial tests and --serial execution continue to run in declaration order.
• The default and TAP reporters include the seed so failures can be reproduced.
• CLI and configuration docs describe the new options.

Fixes #595.

Validation

• npx tap test-tap/runner.js
• npx tap test-tap/integration/assorted.js
• npx xo
• npx tsc --noEmit
• env AVA_FAKE_SCM_ROOT=.fake-root AVA_FORCE_CI=ci node ../../entrypoints/cli.js --tap --seed=ava-seed randomize-tests/test.js
• npm test

npm test completed with the self-hosted AVA suite passing 210 tests with 4 skips, and the TAP suite passing 1633 tests.

Bounty

This is for the IssueHunt-funded #595 bounty. The implementation covers the requested random ordering, reproducible seed, and reported seed behavior.

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #595: Randomize test runs

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