Add phpunit configuration file

Use the configuration file to pass the location of the tests and
autoload class files.

Pass -sp to phpcs command

phpunit.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit
+		bootstrap="vendor/autoload.php"
+		beStrictAboutTestsThatDoNotTestAnything="true"
+		beStrictAboutOutputDuringTests="true">
+
+    <testsuites>
+        <testsuite name="Test suite">
+            <directory>tests/php</directory>
+        </testsuite>
+    </testsuites>
+
+    <filter>
+        <whitelist>
+            <directory suffix=".php">src/</directory>
+        </whitelist>
+    </filter>
+</phpunit>