retrace_restricted_traces: Check for json file. Validate trace upgrades by looking for existing json file instead context header, which was incorrect when traces did not contain a context == 1. Introduce get_trace_json_path function. Use python fstrings. Bug: angleproject:7107 Change-Id: Ib83acb646fb2d9c38d8ca99766170e6c9c7eeb06 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3571884 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>