Make it possible to score arbitrary wptreport.json files

[gsnedders]

This might be more possible via @jgraham's https://github.com/jgraham/wpt-interop, but it would be good to be able to score an arbitrary wptreport.json file, rather than having to wait for results to get onto wpt.fyi and then be able to consumed by the scripts here.

It would be good to have the confidence that the score exactly matches what this script produces, nevertheless.

[jgraham]

Thta repo does in fact already support scoring individual wptreport.json files, or groups of them.