Issue #3502199: Create a new Unit test for LtrimFilter filter plugin

Related to #3502199

tests/src/Unit/Filters/LtrimFilterTest.php

+<?php
+
+namespace Drupal\Tests\fapi_validation\Unit\Filters;
+
+use Drupal\fapi_validation\Plugin\FapiValidationFilter\LtrimFilter;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * Tests filter LtrimFilter.
+ *
+ * @group fapi_validation
+ * @group fapi_validation_filters
+ */
+class LtrimFilterTest extends UnitTestCase {
+
+  /**
+   * Testing valid string.
+   */
+  public function testValidString() {
+    $plugin = new LtrimFilter();
+    $this->assertEquals('test    ', $plugin->filter('   test    '));
+  }
+
+}