✅ Add first tests

tests/backup/FilesTest.php

@@ -0,0 +1,43 @@
+<?php declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+use Shikiryu\Backup\Backup\Files;
+use Shikiryu\Exceptions\BackupException;
+
+class FilesTest extends TestCase
+{
+    public function testIsValid()
+    {
+        $files = [
+            __DIR__ . '/file1',
+            __DIR__ . '/file2',
+        ];
+        foreach ($files as $file) {
+            touch($file);
+        }
+        $file = new Files([
+            'files' => $files
+        ]);
+        $this->assertTrue($file->isValid());
+        foreach ($files as $file) {
+            unlink($file);
+        }
+    }
+
+    public function testIsNotValid()
+    {
+        $file = new Files([
+            'files' => [
+                '/file1',
+                '/file2',
+            ],
+        ]);
+        $this->assertFalse($file->isValid());
+    }
+
+    public function testInvalidConstruct()
+    {
+        $this->expectException(BackupException::class);
+        $file = new Files([]);
+    }
+}