Fix OrderReporter producing empty test_order.log in parallel DRb mode

[gmalette]

Situation

OrderReporter writes test identifiers in its before_test method, but in Rails parallel DRb mode (forked processes with parallelize(workers: N, with: :processes)), before_test is never called. The parallelization server only calls prerecord and record on the parent process's reporters. Since tests execute in forked workers via Minitest.run_one_method (which bypasses reporters entirely), test_order.log is truncated on start and never written to.

This makes it impossible to bisect order-dependent flaky test failures because the test order is never recorded.

Execution

The reporter API can't solve this alone: in DRb mode, prerecord/record run in the parent process with no worker identity, so we can't produce per-worker files from there.

Instead, we prepend a TestOrderTracking module onto Minitest::Test#before_setup, which fires inside the forked worker during run_one_method. There, Process.pid correctly identifies the worker.

Each forked worker writes to its own file (test_order.worker-{pid}.log) with sync writes enabled, since workers never call report and buffered writes risk data loss on crash. The parent's test_order.log is used in non-parallel mode where the parent runs tests directly. Stale worker files from previous runs are cleaned up on start.

To find which worker ran a failing test: grep test_name log/test_order.worker-*.log