api tests: avoid rootPath

extensions/vscode-api-tests/src/singlefolder-tests/workspace.test.ts

@@ -75,7 +75,7 @@ suite('workspace-namespace', () => {
 	});
 
 	test('openTextDocument, untitled is dirty', function () {
-		return vscode.workspace.openTextDocument(vscode.Uri.parse('untitled:' + join(vscode.workspace.rootPath || '', './newfile.txt'))).then(doc => {
+		return vscode.workspace.openTextDocument(vscode.Uri.parse('untitled:' + join(vscode.workspace.workspaceFolders![0].uri.toString() || '', './newfile.txt'))).then(doc => {
 			assert.equal(doc.uri.scheme, 'untitled');
 			assert.ok(doc.isDirty);
 		});