Improve tests coverage for MathPng

Change-Id: Ibb5138512afd20304983be44e0ab45a244634b4b

tests/phpunit/MathPngTest.php

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * @covers MathPng
+ *
+ * @license GPL-2.0-or-later
+ */
+class MathPngTest extends MediaWikiTestCase {
+
+	/** @var string The fallback image HTML tag */
+	const TEST_DUMMY = '<img src="test.png" />';
+
+	public function testConstructor() {
+		$renderer = new MathPng( 'a' );
+
+		$this->assertEquals( 'png', $renderer->getMode() );
+	}
+
+	public function testOutput() {
+		$renderer = $this->getMockBuilder( MathPng::class )
+			->setMethods( [ 'getFallbackImage' ] )
+			->getMock();
+		$renderer->method( 'getFallbackImage' )
+			->willReturn( self::TEST_DUMMY );
+
+		$this->assertSame( self::TEST_DUMMY, $renderer->getHtmlOutput() );
+	}
+
+}