I18nTextDomainFixer: add a test for code handling STDIN

The sniff bails early when handling a plugin header passed via STDIN. This commit adds a test to cover that part of the sniff code.

WordPress/Tests/Utils/I18nTextDomainFixerStdInTest.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="I18nTextDomainFixerStdInTest" xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/PHPCSStandards/PHP_CodeSniffer/master/phpcs.xsd">
+	<rule ref="./WordPress/Sniffs/Utils/I18nTextDomainFixerSniff.php"/>
+</ruleset>

WordPress/Tests/Utils/I18nTextDomainFixerUnitTest.php

@@ -9,6 +9,9 @@
 
 namespace WordPressCS\WordPress\Tests\Utils;
 
+use PHP_CodeSniffer\Files\DummyFile;
+use PHP_CodeSniffer\Ruleset;
+use PHP_CodeSniffer\Tests\ConfigDouble;
 use PHP_CodeSniffer\Tests\Standards\AbstractSniffUnitTest;
 use PHPCSUtils\BackCompat\Helper;
 
@@ -197,4 +200,30 @@ public function getWarningList( $testFile = '' ) {
 				return array();
 		}
 	}
+
+	/**
+	 * Test the sniff bails early when handling a plugin header passed via STDIN.
+	 *
+	 * @return void
+	 */
+	public function testStdIn() {
+		$config            = new ConfigDouble();
+		$config->standards = array( __DIR__ . '/I18nTextDomainFixerStdInTest.xml' );
+
+		$ruleset = new Ruleset( $config );
+
+		$content = '<?php // phpcs:set WordPress.Utils.I18nTextDomainFixer new_text_domain test-std-in
+/**
+ * Plugin Name: Missing text domain, docblock format.
+ * Plugin URI: https://www.bigvoodoo.com/
+ * Description: Sniff triggers a missing text domain error for a normal file, but not for STDIN.
+ */';
+
+		$file = new DummyFile( $content, $ruleset, $config );
+		$file->process();
+
+		$this->assertSame( 0, $file->getErrorCount() );
+		$this->assertSame( 0, $file->getWarningCount() );
+		$this->assertCount( 0, $file->getErrors() );
+	}
 }