Add file to control the settings that PR carrot tests run under

Unfortunately I don't have good docs for this file. Some other examples are https://lab.civicrm.org/extensions/cdntaxreceipts/-/blob/master/tests/civicarrot.json and https://lab.civicrm.org/DaveD/miscminktests/-/blob/master/tests/civicarrot.json.

But a quick runthrough of what this PR has:

• civicrm - I don't really like how it defaults to RC, but that's why there's fails right now, because there is no RC right now there is just 5.53 and master. I've picked dev-master since that makes more sense to me for runs against PRs.
  • There is also CIVICARROT_CIVI_RELEASECANDIDATE and CIVICARROT_CIVI_LATEST if you want the RC or the latest release. Can also pick specific composer numbers like ~5.51.0.
• php - defaults to something sensible - currently 7.4. Can specify a number if desired but this isn't composer numbers it's just regular numbers, e.g. 8.0, since composer can't even exist until php is installed.
• drupal - at the moment packagist is returning drupal 10 if you say "LATEST", so in this case PRIOR means drupal 9. I need to work that out but right now PRIOR is the only thing that makes sense, or alternatively ^9. Drupal 7 won't work in this context.