Add test

tests/Modifiers/SpecializableModifierTest.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Intervention\Image\Tests\Modifiers;
+
+use Intervention\Image\Modifiers\SpecializableModifier;
+use Intervention\Image\Interfaces\ImageInterface;
+use Intervention\Image\Tests\TestCase;
+use Mockery;
+
+class SpecializableModifierTest extends TestCase
+{
+    public function testApply(): void
+    {
+        $modifier = Mockery::mock(SpecializableModifier::class)->makePartial();
+        $image = Mockery::mock(ImageInterface::class);
+        $image->shouldReceive('modify')->andReturn($image);
+
+        $result = $modifier->apply($image);
+        $this->assertInstanceOf(ImageInterface::class, $result);
+    }
+}