Add PHPUnit configuration file for version 9

- Created a new phpunit-9.xml file to configure PHPUnit for testing.
- Set up bootstrap file and defined test suite directory.
- Excluded SampleTest.php from the test suite.

Author: giacomo-secchi

composer.json

@@ -9,7 +9,7 @@
     "require-dev": {
         "wp-cli/i18n-command": "^2.6",
         "wp-coding-standards/wpcs": "^3.2",
-        "phpunit/phpunit": "^11.5",
+        "phpunit/phpunit": "^9.6",
         "yoast/phpunit-polyfills": "^4.0",
         "brain/monkey": "^2.6",
         "phpcompatibility/phpcompatibility-wp": "^2.1"
@@ -23,7 +23,7 @@
 			"@php ./vendor/bin/phpcbf --report=summary,source"
 		],
         "test": [
-			"@php ./vendor/bin/phpunit  --testdox"
+			"@php ./vendor/bin/phpunit  -c phpunit-9.xml --testdox"
 		],
         "coverage": [
 			"@php ./vendor/bin/phpunit --coverage-html ./coverage-report"