Extract a base test case, move mock config

tests/Glassboxx/TestCase.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Opdavies\Glassboxx\Tests\Glassboxx;
+
+use Opdavies\Glassboxx\Config;
+
+class TestCase extends \PHPUnit\Framework\TestCase
+{
+    /** @var Config */
+    protected $config;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->config = $this->getMockBuilder(Config::class)
+            ->onlyMethods([])
+            ->setConstructorArgs(
+                [
+                    'vendor_id' => 123,
+                    'username' => 'opdavies',
+                    'password' => 'secret',
+                ]
+            )
+            ->getMock();
+    }
+}