Initially produced using:

  for d in "test cases/failing/"* ; do rm -r _build ; ./meson.py setup "$d" _build | grep ERROR >"$d"/expected_stdout.txt; done

then converted to json with jq using:

  jq --raw-input --slurp 'split("\n") | {stdout: map({line: select(. != "")})}' expected_stdout.txt >test.json

or merged with existing json using:

  jq --slurp '.[0] + .[1]' test.json expected.json >test.json.new

v2:
Add some comments to explain the match when it isn't totally obvious

v3:
Add or adjust existing re: in expected output to handle '/' or '\' path
separators appearing in message, not location.

v4:
Put expected stdout in test.json, rather than a separate expected_stdout.txt file
Park comments in an unused 'comments' key, as JSON doesn't have a syntax for comments