PHPLIB-462: Allow installing the latest PHPUnit release

.gitignore

@@ -6,3 +6,4 @@ vendor/
 # PHPUnit
 phpunit.phar
 phpunit.xml
+.phpunit.result.cache

composer.json

@@ -16,7 +16,7 @@
         "ext-mongodb": "^1.6"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7.27 || ^6.4"
+        "phpunit/phpunit": "^5.7.27 || ^6.4 || ^8.3"
     },
     "autoload": {
         "psr-4": { "MongoDB\\": "src/" },

phpunit.xml.dist

@@ -2,15 +2,10 @@
 
 <phpunit
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/4.3/phpunit.xsd"
-         backupGlobals="false"
-         backupStaticAttributes="false"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/8.3/phpunit.xsd"
+         beStrictAboutOutputDuringTests="true"
+         beStrictAboutChangesToGlobalState="true"
          colors="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
-         stopOnFailure="false"
-         syntaxCheck="false"
          bootstrap="tests/bootstrap.php"
 >