Providing unique identifiers for files and directories created as part of unit tests. Bug: webrtc:15833 Change-Id: If2835c362c47a111aa99b0e3c6ad6a33be061978 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/338260 Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Harald Alvestrand <hta@webrtc.org> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org> Cr-Commit-Position: refs/heads/main@{#41704}