Support generating summary reports when using pytest-xdist

[ConorMacBride]

• When mpl-results-path is not set, ensure temp directory is shared across all workers
• When in a worker:
  • Write JSON summaries to {results_dir}/results-xdist-{test_run_uid}-{worker_id}.json
  • Skip generating HTML reports
• When in the controller:
  • Read all JSON summaries {results_dir}/results-xdist-{test_run_uid}-*.json back into current controller process
  • Write merged JSON summaries back to {results_dir}/results.json
  • Generate HTML reports as usual

This likely won't work for remote xdist workers. Maybe we need a way to detect that and warn?

TODO

• Merge generated hash libraries together also (and test)

Closes #136
Closes #239