This modifies the handling of multiline message reporters, where more than one pattern have a message field.

For instance

{
  "regexp": "^\\(\\d+\\) .+\\(([^\\)]+)\\): (failed in .* phase)$",
  "file": 1,
  "message": 2
},
{
  "regexp": "^    (.*)",
  "message": 1,
  "loop": true
}

The existing behaviour was to use the first matching message. This PR changes this by appending to an existing message instead (retaining newlines).

One obvious question is whether the new behaviour should be made optional or not.