Added initial unit tests

phpunit.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit bootstrap="vendor/autoload.php" colors="true">
+    <testsuites>
+        <testsuite name="Directory Lister Test Suite">
+            <directory suffix="Test.php">tests/</directory>
+        </testsuite>
+    </testsuites>
+    <filter>
+        <whitelist processUncoveredFilesFromWhitelist="true">
+            <directory suffix=".php">src/</directory>
+        </whitelist>
+    </filter>
+</phpunit>

tests/Unit/Controllers/DirectoryControllerTest.php

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+<?php
+
+namespace Tests\Controllers;
+
+use App\Controllers\DirectoryController;
+use PHLAK\Config\Config;
+use PHPUnit\Framework\TestCase;
+use Psr\Http\Message\ResponseInterface;
+use Slim\Psr7\Response;
+use Slim\Views\Twig;
+use Symfony\Component\Finder\Finder;
+
+class DirectoryControllerTest extends TestCase
+{
+    public function test_it_returns_a_response()
+    {
+        $controller = new DirectoryController(
+            $this->createMock(Config::class),
+            $this->createMock(Twig::class)
+        );
+
+        $response = $controller(
+            $this->createMock(Finder::class),
+            $this->createMock(Response::class),
+            'tests/files'
+        );
+
+        $this->assertInstanceOf(ResponseInterface::class, $response);
+    }
+}

tests/Unit/Support/HelpersTest.php

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+<?php
+
+namespace Tests\Unit\Support;
+
+use PHPUnit\Framework\TestCase;
+
+class HelpersTest extends TestCase
+{
+    public function test_it_can_get_an_environment_variable()
+    {
+        putenv('TEST_STRING=Test string; please ignore');
+
+        $env = env('TEST_STRING');
+
+        $this->assertEquals('Test string; please ignore', $env);
+    }
+
+    public function test_it_can_return_a_default_value()
+    {
+        $env = env('DEFAULT_TEST', 'Test default; please ignore');
+
+        $this->assertEquals('Test default; please ignore', $env);
+    }
+
+    public function test_it_can_a_retrieve_boolean_value()
+    {
+        putenv('TRUE_TEST=true');
+        putenv('FALSE_TEST=false');
+
+        $this->assertTrue(env('TRUE_TEST'));
+        $this->assertFalse(env('FALSE_TEST'));
+    }
+
+    public function test_it_can_retrieve_a_null_value()
+    {
+        putenv('NULL_TEST=null');
+
+        $this->assertNull(env('NULL_TEST'));
+    }
+
+    public function test_it_can_be_surrounded_bys_quotation_marks()
+    {
+        putenv('QUOTES_TEST="Test charlie; please ignore"');
+
+        $env = env('QUOTES_TEST');
+
+        $this->assertEquals('Test charlie; please ignore', $env);
+    }
+}

tests/files/test.txt

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+Test file; please ignore