Added an service for generating Backup codes and added some tests.

tests/Services/TFA/BackupCodeGeneratorTest.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace App\Tests\Services\TFA;
+
+use App\Services\TFA\BackupCodeGenerator;
+use PHPUnit\Framework\TestCase;
+
+class BackupCodeGeneratorTest extends TestCase
+{
+    /**
+     * Test if an exception is thrown if you are using a too high code length
+     */
+    public function testLengthUpperLimit()
+    {
+        $this->expectException(\RuntimeException::class);
+        new BackupCodeGenerator(33, 10);
+    }
+
+    /**
+     * Test if an exception is thrown if you are using a too high code length
+     */
+    public function testLengthLowerLimit()
+    {
+        $this->expectException(\RuntimeException::class);
+        new BackupCodeGenerator(4, 10);
+    }
+
+
+    public function codeLengthDataProvider()
+    {
+        return [[6], [8], [10], [16]];
+    }
+
+    /**
+     * @dataProvider  codeLengthDataProvider
+     */
+    public function testGenerateSingleCode(int $code_length)
+    {
+        $generator = new BackupCodeGenerator($code_length, 10);
+        $this->assertRegExp("/^([a-f0-9]){{$code_length}}\$/", $generator->generateSingleCode());
+    }
+
+    public function codeCountDataProvider()
+    {
+        return [[2], [8], [10]];
+    }
+
+    /**
+     * @dataProvider codeCountDataProvider
+     */
+    public function testGenerateCodeSet(int $code_count)
+    {
+        $generator = new BackupCodeGenerator(8, $code_count);
+        $this->assertCount($code_count, $generator->generateCodeSet());
+    }
+}