[TASK] Configure PHPUnit to be more strict

This is the v8.x backport of #743.

Author: oliverklee

phpunit.xml

@@ -1,6 +1,13 @@
-<phpunit
-    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd">
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/5.7/phpunit.xsd"
+         beStrictAboutChangesToGlobalState="true"
+         beStrictAboutOutputDuringTests="true"
+         beStrictAboutTodoAnnotatedTests="true"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         convertDeprecationsToExceptions="true"
+         forceCoversAnnotation="true"
+         verbose="true">
     <testsuites>
         <testsuite name="Project Test Suite">
             <directory>tests</directory>