Issue #3243405: Add a kernel test of UpdateVersionValidator

tests/src/Kernel/ReadinessValidation/UpdateVersionValidatorTest.php

+<?php
+
+namespace Drupal\Tests\automatic_updates\Kernel\ReadinessValidation;
+
+use Drupal\automatic_updates\Validation\ValidationResult;
+use Drupal\Tests\automatic_updates\Kernel\AutomaticUpdatesKernelTestBase;
+
+/**
+ * @covers \Drupal\automatic_updates\Validator\UpdateVersionValidator
+ *
+ * @group automatic_updates
+ */
+class UpdateVersionValidatorTest extends AutomaticUpdatesKernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'automatic_updates',
+    'package_manager',
+  ];
+
+  /**
+   * Tests an update version that is same major & minor version as the current.
+   */
+  public function testNoMajorOrMinorUpdates(): void {
+    $this->assertCheckerResultsFromManager([], TRUE);
+  }
+
+  /**
+   * Tests an update version that is a different major version than the current.
+   */
+  public function testMajorUpdates(): void {
+    $this->setCoreVersion('8.9.1');
+    $result = ValidationResult::createError(['Updating from one major version to another is not supported.']);
+    $this->assertCheckerResultsFromManager([$result], TRUE);
+
+  }
+
+  /**
+   * Tests an update version that is a different minor version than the current.
+   */
+  public function testMinorUpdates(): void {
+    $this->setCoreVersion('9.7.1');
+    $result = ValidationResult::createError(['Updating from one minor version to another is not supported.']);
+    $this->assertCheckerResultsFromManager([$result], TRUE);
+  }
+
+}