After a full mix test, compares ExUnit's failures manifest (the file
mix test --failed reads) with a selection saved by mix affected.files --format json. Run with MIX_ENV=test.
mix affected.report --selection selection.json [--format text|json] [--output path]JSON: schema_version, selection_full_run, failed_files,
missed_files (failed files not in the selection; always [] for a
full-run selection), selected_count, total_test_files. Text: the missed
files, one per line.
Exit status 0 whatever the misses (shadow data); 1 on a usage error,
including an unreadable selection file. An unknown or missing failures
manifest reports failed_files: [] and warns on stderr.