All tests require an image that exists. It will not cause the 404 error.

tests/ckeditor.js

@@ -154,7 +154,7 @@ describe( 'DecoupledEditor build', () => {
 			} );
 
 			it( 'image works', () => {
-				const data = '<figure class="image"><img src="./manual/sample.jpg"></figure>';
+				const data = '<figure class="image"><img src="/assets/sample.png"></figure>';
 
 				editor.setData( data );
 				expect( editor.getData() ).to.equal( data );