PHPUnit config: display errors/warnings/notices etc

The config as it was, was hiding 6 `Undefined array key`/`Trying to access array offset on null` notices.

As any notice caused by a sniff will stop a PHPCS scan of a file dead, these should be discovered when running the tests and then fixed.
Better yet: the tests should fail on them. This change in the configuration makes it so.

Note: this does mean you now have a bug to fix.

Also see: squizlabs/PHP_CodeSniffer#3844

phpunit.xml.dist

@@ -13,6 +13,13 @@
 	stopOnDefect="true"
 	executionOrder="defects"
 	cacheDirectory="temp/phpunit-cache"
+	displayDetailsOnTestsThatTriggerErrors="true"
+	displayDetailsOnTestsThatTriggerWarnings="true"
+	displayDetailsOnTestsThatTriggerNotices="true"
+	displayDetailsOnTestsThatTriggerDeprecations="true"
+	failOnWarning="true"
+	failOnNotice="true"
+	failOnDeprecation="true"
 >
 	<testsuites>
 		<testsuite name="Slevomat Coding Standard">