test: 加入 integration 測試套件（連真實 MariaDB）

從 wp-data-optimizer v3.4.6 移植 1 個 integration 測試檔，並新增
tests/integration/bootstrap.php（複用核心的 integration bootstrap，不重複
 與 WP stub）＋ phpunit-integration.xml ＋ CI integration job。

phpunit-integration.xml

@@ -0,0 +1,15 @@
+<?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="tests/integration/bootstrap.php"
+    colors="true"
+    beStrictAboutOutputDuringTests="true"
+    failOnWarning="true"
+>
+    <testsuites>
+        <testsuite name="woocommerce-addon-integration">
+            <directory>tests/integration</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>