[Feature] Include `parallelIndex` in the JSON report

[arjunattam]

Hi team! 😁 This is a follow-up to #19570 and #19519

My goal is to use the JSON report output to build a visualization that shows worker utilization (similar to the image in #19519). I believe this can be achieved by adding the parallelIndex property to JSONReportTestResult. Is there a different way to achieve the goal?

export interface JSONReportTestResult {
  workerIndex: number;
+ parallelIndex: number;
  status: TestStatus | undefined;
  ...
}

If this change sounds good, I can submit a PR. Thanks!