Use image from test resources on github instead of using external one

Fixes #2246

Author: iloveeclipse

ua/org.eclipse.tips.tests/src/org/eclipse/tips/core/TipImageURLTest.java

@@ -25,7 +25,7 @@
 
 public class TipImageURLTest {
 
-	private static final String URL = "http://remainsoftware.com/img.png";
+	private static final String URL = "https://github.com/eclipse-platform/eclipse.platform/blob/master/ua/org.eclipse.tips.tests/icons/dummy.png";
 
 	@org.junit.jupiter.api.Test
 	public void testTipImage() {