Add tests for no stable tag in readme check

tests/phpunit/testdata/plugins/test-plugin-plugin-readme-errors-no-stable-tag/load.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * Plugin Name: Test Plugin Readme Errors (no stable tag)
+ * Plugin URI: https://github.com/WordPress/plugin-check
+ * Description: Test plugin for the Readme check.
+ * Requires at least: 6.0
+ * Requires PHP: 5.6
+ * Version: n.e.x.t
+ * Author: WordPress Performance Team
+ * Author URI: https://make.wordpress.org/performance/
+ * License: GPLv2 or later
+ * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+ * Text Domain: test-plugin-check-errors
+ *
+ * @package test-plugin-check-errors
+ */

tests/phpunit/testdata/plugins/test-plugin-plugin-readme-errors-no-stable-tag/readme.txt

@@ -0,0 +1,11 @@
+=== Test Plugin with readme ===
+
+Contributors:      plugin-check
+Requires at least: 6.0
+Tested up to:      6.1
+Requires PHP:      5.6
+License:           GPLv2 or later
+License URI:       https://www.gnu.org/licenses/gpl-2.0.html
+Tags:              testing, security
+
+Plugin description.

tests/phpunit/tests/Checker/Checks/Plugin_Readme_Check_Tests.php

@@ -75,6 +75,30 @@ public function test_run_with_errors_stable_tag() {
 		$this->assertEquals( 'stable_tag_mismatch', $errors['readme.txt'][0][0][1]['code'] );
 	}
 
+	public function test_run_with_errors_no_stable_tag() {
+		$readme_check  = new Plugin_Readme_Check();
+		$check_context = new Check_Context( UNIT_TESTS_PLUGIN_DIR . 'test-plugin-plugin-readme-errors-no-stable-tag/load.php' );
+		$check_result  = new Check_Result( $check_context );
+
+		$readme_check->run( $check_result );
+
+		$errors   = $check_result->get_errors();
+		$warnings = $check_result->get_warnings();
+
+		$this->assertNotEmpty( $errors );
+		$this->assertArrayHasKey( 'readme.txt', $errors );
+		$this->assertEquals( 1, $check_result->get_error_count() );
+
+		$this->assertEmpty( $warnings );
+		$this->assertEquals( 0, $check_result->get_warning_count() );
+
+		// Check for no stable tag error.
+		$this->assertArrayHasKey( 0, $errors['readme.txt'] );
+		$this->assertArrayHasKey( 0, $errors['readme.txt'][0] );
+		$this->assertArrayHasKey( 'code', $errors['readme.txt'][0][0][0] );
+		$this->assertEquals( 'no_stable_tag', $errors['readme.txt'][0][0][0]['code'] );
+	}
+
 	public function test_run_with_errors_license() {
 		$readme_check  = new Plugin_Readme_Check();
 		$check_context = new Check_Context( UNIT_TESTS_PLUGIN_DIR . 'test-plugin-plugin-readme-errors-license/load.php' );