[symfony/panther] Set warn_if_missing=true when trying to add lines to phpunit.xml.dist

[nicolas-grekas]

Q	A
License	MIT
Doc issue/PR	-

[github-actions]

Thanks for the PR 😍

How to test these changes in your application

1. Define the SYMFONY_ENDPOINT environment variable:

   # On Unix-like (BSD, Linux and macOS)
   export SYMFONY_ENDPOINT=https://raw.githubusercontent.com/symfony/recipes/flex/pull-1216/index.json
   # On Windows
   SET SYMFONY_ENDPOINT=https://raw.githubusercontent.com/symfony/recipes/flex/pull-1216/index.json

2. Install the package(s) related to this recipe:

   composer req 'symfony/flex:^1.16'
   composer req 'symfony/panther:^1.0'

3. Don't forget to unset the SYMFONY_ENDPOINT environment variable when done:

   # On Unix-like (BSD, Linux and macOS)
   unset SYMFONY_ENDPOINT
   # On Windows
   SET SYMFONY_ENDPOINT=

Diff between recipe versions

In order to help with the review stage, I'm in charge of computing the diff between the various versions of patched recipes.
I'm going keep this comment up to date with any updates of the attached patch.