Add phpunit config

.gitignore

@@ -1,2 +1,3 @@
 /vendor/
 /.php-cs-fixer.cache
+/.phpunit.cache

phpunit.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         failOnWarning="true"
+         failOnRisky="true"
+         beStrictAboutOutputDuringTests="true"
+         cacheDirectory=".phpunit.cache">
+  <testsuites>
+    <testsuite name="Unit">
+      <directory>tests/Unit</directory>
+    </testsuite>
+  </testsuites>
+  <source>
+    <include>
+      <directory>src</directory>
+    </include>
+  </source>
+</phpunit>