fix: aggregate custom reporter results in run-workers --by suite (#5411)

[mirao]

Summary

• When using run-workers with --by suite, custom reporters (mochawesome, mocha-junit-reporter, etc.) only contained data from the last worker because each worker independently ran the reporter and overwrote the same output files
• Strip custom reporters from worker configs and replay aggregated results through the reporter in the main thread after all workers complete
• Handles both single-output and config.multiple (per-browser) scenarios with correct path resolution for nested reporter options

Test plan

• Unit test verifies _runReporters() replays aggregated results through a custom reporter
• Acceptance test verifies mochawesome JSON + HTML reports contain all suites when using --by suite
• Acceptance test verifies per-browser reports with config.multiple each contain all suites
• All existing worker unit tests pass (11 passing)
• All existing worker runner tests pass (31 passing)

Fixes #5411

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