Increase test coverage: Array.prototype.toString

Branch coverage:
Before: 5/6
After:  6/6

JerryScript-DCO-1.0-Signed-off-by: Mate Dabis mdabis@inf.u-szeged.hu

Author: matedabis

tests/jerry/array-prototype-tostring.js

@@ -61,3 +61,11 @@ try {
   assert (e.message === "foo");
   assert (e instanceof ReferenceError);
 }
+
+// Checking behavior when the function's this_argument is undefined
+try {
+  Array.prototype.toString.call(undefined);
+  assert(false);
+} catch (e) {
+  assert(e instanceof TypeError);
+}