Add --randomize flag to randomize test order

[stevenmini2019]

Summary

Implements seeded randomization of the test run order, resolving #595.

• New --randomize flag randomizes both the order of test files and the order of concurrent tests within a file.

• New --seed <seed> flag reproduces a specific order. When --randomize is used without --seed, a random 16-character hex seed is generated.

• The active seed is reported in the run output so a failing order can be reproduced:

  Randomized test order with seed <seed>. Re-run with --seed <seed> to reproduce the same order.
  

• .serial tests are unaffected: within-file randomization only reorders concurrent tests, preserving the source order that serial execution relies on.

• Passing an empty --seed value exits with a clear error.

Implementation

• lib/test-order.js (new): deterministic Fisher-Yates shuffle backed by an xorshift32 PRNG seeded from an FNV-1a hash of the seed string. fileOrderSeed() and testOrderSeed() give file-level and within-file salts so the two shuffle passes are independent.
• lib/api.js: derives (or accepts) randomSeed when randomize is enabled, shuffles the selected files, and includes randomSeed in the run event.
• lib/runner.js: shuffles concurrent tests within a file using a file-specific salt.
• lib/worker/base.js: forwards randomSeed into the worker so the runner can use it.
• lib/cli.js: adds the --randomize and --seed flags and resolves randomize/randomSeed.
• lib/reporters/default.js: prints the seed line when a run is randomized.
• test/scheduler/test.js: tests for seed reporting, generated seed, absence without --randomize, and the empty --seed error.

Notes

• Default behavior is unchanged: without --randomize, no shuffling occurs and nothing is reported.
• The same seed always yields the same order across machines and runs.