[TESTS] Running in CI should not create but rather fail on missing snapshots (#1039)

* tests: running in CI should not create but rather fail on missing snapshots

Otherwise they get created on the CI infrastructure but that's it, the
build won't fail although it was forgotten to add the snapshot.

* Bump minimum phpunit version which correctly accepts the `-d` parameter

The `prefer-lowest` version we get with L6 is 8.0.0 which throws this error:
```
PHP Fatal error:  Uncaught TypeError: ini_set() expects parameter 2 to be string, bool given in /home/runner/work/laravel-ide-helper/laravel-ide-helper/vendor/phpunit/phpunit/src/TextUI/Command.php:379
```

* Update composer.json

Co-authored-by: Barry vd. Heuvel <[email protected]>

composer.json

@@ -36,6 +36,7 @@
         "illuminate/view": "^6 || ^7 || ^8",
         "mockery/mockery": "^1.3",
         "orchestra/testbench": "^4 || ^5 || ^6",
+        "phpunit/phpunit": "^8.5 || ^9",
         "spatie/phpunit-snapshot-assertions": "^1.4 || ^2.2 || ^3",
         "vimeo/psalm": "^3.12"
     },
@@ -75,6 +76,7 @@
             "php-cs-fixer fix --config=.php_cs.tests.php"
         ],
         "test": "phpunit",
+        "test-ci": "phpunit -d --without-creating-snapshots",
         "test-regenerate": "phpunit -d --update-snapshots"
     }
 }