Upgrade to PHPUnit v9

.gitignore

@@ -1,5 +1,6 @@
 /.build/
 /node_modules/
 /vendor/
+.phpunit.result.cache
 composer.lock
 package-lock.json

composer.json

@@ -26,7 +26,7 @@
     },
     "require-dev": {
         "monolog/monolog": "^2.0",
-        "phpunit/phpunit": "^6.5|^7.0",
+        "phpunit/phpunit": "^9.0",
         "symfony/process": "^4.0|^5.0"
     },
     "autoload": {

phpunit.xml

@@ -1,20 +1,9 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/6.5/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
          bootstrap="vendor/autoload.php"
-         forceCoversAnnotation="true"
-         beStrictAboutCoversAnnotation="true"
-         beStrictAboutOutputDuringTests="true"
-         beStrictAboutTodoAnnotatedTests="true"
-         colors="true"
-         verbose="true">
-    <testsuite name="default">
-        <directory suffix="Test.php">tests</directory>
-    </testsuite>
-
-    <filter>
-        <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">src</directory>
-        </whitelist>
-    </filter>
+         colors="true">
+  <testsuite name="default">
+    <directory suffix="Test.php">tests</directory>
+  </testsuite>
 </phpunit>