Add passing unit tests

assets/hello.txt

@@ -0,0 +1 @@
+SGVsbG8gV29ybGQh

tests/new_unit_tests/Commands/ArtCommandTest.php

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+<?php
+namespace Pantheon\Terminus\UnitTests\Commands;
+
+use Pantheon\Terminus\Tests\CommandTestCase;
+use Symfony\Component\Console\Output\BufferedOutput;
+
+class ArtCommandTest extends CommandTestCase
+{
+
+    public function setUp()
+    {
+        parent::setUp();
+        $this->setInput(['command' => 'art', 'name' => 'hello']);
+    }
+
+    public function testArtPrintsArt()
+    {
+        $this->assertEquals('Hello World!', $this->runCommand()->getOutput());
+    }
+
+    public function testArtRejectsNonExistantFiles()
+    {
+        $this->setInput(['command' => 'art', 'name' => 'foo']);
+        $this->assertEquals('Not a valid work of art!', $this->runCommand()->getOutput());
+    }
+}