Issue #3500208

1. Added test for test functionality of the filter machine_name.

Related to #3502200

tests/src/Unit/Filters/MachineNameFilterTest.php

+<?php
+
+namespace Drupal\Tests\fapi_validation\Unit\Filters;
+
+use Drupal\fapi_validation\FapiValidationFiltersManager;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests filter MachineNameFilterTest.
+ *
+ * @group fapi_validation
+ * @group fapi_validation_filters
+ */
+class MachineNameFilterTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['fapi_validation'];
+
+  /**
+   * The filter plugin manager.
+   *
+   * @var \Drupal\fapi_validation\FapiValidationFiltersManager
+   */
+  protected FapiValidationFiltersManager $fapiValidationFiltersManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+    $this->fapiValidationFiltersManager = $this->container->get('plugin.manager.fapi_validation_filters');
+  }
+
+  /**
+   * Testing machine_name`s filter plugin.
+   */
+  public function testValidString() : void {
+    $plugin = $this->fapiValidationFiltersManager->createInstance('machine_name');
+    $this->assertEquals('test_test', $plugin->filter('TesT test'));
+  }
+
+}