added assertion to tests

ts/typings/assertion-error/assertion-error.d.ts

@@ -0,0 +1,15 @@
+// Type definitions for assertion-error 1.0.0
+// Project: https://github.com/chaijs/assertion-error
+// Definitions by: Bart van der Schoor <https://github.com/Bartvds>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+
+declare module 'assertion-error' {
+	class AssertionError implements Error {
+		constructor(message: string, props?: any, ssf?: Function);
+		name: string;
+		message: string;
+		showDiff: boolean;
+		stack: string;
+	}
+	export = AssertionError;
+}