Add TenancyTestCase base class for tenant-aware test initialization and central context support

- Adds a reusable base test case to simplify writing tenant-aware tests
- Automatically initializes tenancy using TEST_TENANT from config or .env.testing
- Provides `runInCentralContext()` to temporarily switch to central DB context
- Improves test reliability and consistency for multi-tenant applications

assets/config.php

@@ -196,4 +196,6 @@ return [
         '--class' => 'DatabaseSeeder', // root seeder class
         // '--force' => true, // This needs to be true to seed tenant databases in production
     ],
+
+    'test_tenant' => env('TEST_TENANT', 1),
 ];