test: add failing authenticate user test

Red phase: write AuthenticateUserTest with cases for valid credentials, empty email/password (null and empty string), unknown email, wrong password, and fresh instance guarantee. Fakes included.

backend/phpunit.xml

@@ -0,0 +1,19 @@
+<?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"
+         cacheDirectory=".phpunit.cache"
+         executionOrder="depends,defects"
+
+         colors="true">
+    <testsuites>
+        <testsuite name="unit">
+            <directory>tests/Unit</directory>
+        </testsuite>
+    </testsuites>
+    <source>
+        <include>
+            <directory>app</directory>
+        </include>
+    </source>
+</phpunit>