Add XSD schema to phpunit.xml.dist config

- Add XSD schema, so it can be validated (and PHPStorm provides
  auto-completion)
- Drop backupGlobals option, because false is the default value

composer.json

@@ -23,7 +23,7 @@
         "moontoast/math": "^1.1",
         "paragonie/random-lib": "^2",
         "php-mock/php-mock-phpunit": "^2.5",
-        "phpunit/phpunit": "^8",
+        "phpunit/phpunit": "^8.5",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "suggest": {

phpunit.xml.dist

@@ -1,7 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit bootstrap="./tests/bootstrap.php" colors="true" verbose="true" backupGlobals="false">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/8.5/phpunit.xsd"
+        bootstrap="./tests/bootstrap.php"
+        colors="true"
+        verbose="true">
     <testsuites>
-        <testsuite>
+        <testsuite name="ramsey/uuid test suite">
             <directory>./tests</directory>
         </testsuite>
     </testsuites>