Issue #3502217: Create a new Unit test for LengthValidator validation plugin

Related to #3502217

tests/src/Unit/Validators/LengthValidatorTest.php

+<?php
+
+namespace Drupal\Tests\fapi_validation\Unit\Validators;
+
+use Drupal\fapi_validation\Plugin\FapiValidationValidator\LengthValidator;
+use Drupal\fapi_validation\Validator;
+
+/**
+ * Tests Length Validator.
+ *
+ * @group fapi_validation
+ * @group fapi_validation_validators
+ */
+class LengthValidatorTest extends BaseValidator {
+
+  /**
+   * Alpha validation.
+   *
+   * @var \Drupal\fapi_validation\FapiValidationValidatorsInterface
+   */
+  protected $plugin;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp(): void {
+    parent::setUp();
+
+    $this->plugin = new LengthValidator();
+  }
+
+  /**
+   * Testing valid string.
+   */
+  public function testValidString() {
+    $validator = new Validator('length[5, *]', 'dljshfkjshf');
+
+    $this->assertTrue($this->plugin->validate($validator, [], $this->decoratedFormState));
+  }
+
+  /**
+   * Testing invalid string.
+   */
+  public function testInvalidString() {
+    $validator = new Validator('length[5, *]', 'gwe');
+
+    $this->assertFalse($this->plugin->validate($validator, [], $this->decoratedFormState));
+  }
+
+}