Updated link to PHPUnit's xsd to use local one

phpunit-api.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.4/phpunit.xsd"
+    xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
     bootstrap="./config/test/bootstrap_api_tests.php"
     colors="true"
 >

phpunit-db.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.4/phpunit.xsd"
+    xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
     bootstrap="./config/test/bootstrap_db_tests.php"
     colors="true"
 >

phpunit.xml.dist

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.4/phpunit.xsd"
+    xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
     bootstrap="./vendor/autoload.php"
     colors="true"
 >