Add basic tests for ULS' ResourceLoader modules

This is a starting point to increase the test coverage in this code
base.

Change-Id: I215713e928e9b55615afd17ccf7a7082ef559e01

tests/phpunit/ULSJsonMessageLoaderTest.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace UniversalLanguageSelector\Tests;
+
+use ULSJsonMessageLoader;
+
+/**
+ * @covers \ULSJsonMessageLoader
+ *
+ * @license GPL-2.0-or-later
+ * @author Thiemo Kreuz
+ */
+class ULSJsonMessageLoaderTest extends \PHPUnit\Framework\TestCase {
+	use \PHPUnit4And6Compat;
+
+	public function testGetFilenamesWithBadInput() {
+		$instance = new ULSJsonMessageLoader();
+
+		$this->setExpectedException( \MWException::class );
+		$instance->getFilenames( null );
+	}
+
+	public function testGetMessagesWithBadInput() {
+		$instance = new ULSJsonMessageLoader();
+
+		$this->setExpectedException( \MWException::class );
+		$instance->getMessages( null );
+	}
+
+	public function testWithInvalidLanguageCode() {
+		$instance = new ULSJsonMessageLoader();
+		$languageCode = '0';
+
+		$filenames = $instance->getFilenames( $languageCode );
+		$this->assertSame( [], $filenames );
+
+		$messages = $instance->getMessages( $languageCode );
+		$this->assertSame( [], $messages );
+	}
+
+	public function testWithValidLanguageCode() {
+		$instance = new ULSJsonMessageLoader();
+		$languageCode = 'en';
+
+		$filenames = $instance->getFilenames( $languageCode );
+		$this->assertContainsOnly( 'string', $filenames );
+
+		$messages = $instance->getMessages( $languageCode );
+		unset( $messages['@metadata'] );
+		$this->assertContainsOnly( 'string', array_keys( $messages ) );
+		$this->assertContainsOnly( 'string', $messages );
+	}
+
+}