Promise.prototype.finally: add more tests

- per https://github.com/tc39/ecma262/pull/1083#issuecomment-361775023

test/built-ins/Promise/prototype/finally/invokes-then-with-function.js

@@ -27,6 +27,7 @@ target.then = function(a, b) {
 };
 
 var originalFinallyHandler = function () {};
+var anonName = Object(function () {}).name;
 
 var result = Promise.prototype.finally.call(target, originalFinallyHandler, 2, 3);
 
@@ -43,11 +44,16 @@ assert.sameValue(
   'Invokes `then` method with a function as the first argument'
 );
 assert.notSameValue(firstArg, originalFinallyHandler, 'Invokes `then` method with a different fulfillment handler');
+assert.sameValue(firstArg.length, 1, 'fulfillment handler has a length of 1');
+assert.sameValue(firstArg.name, anonName, 'fulfillment handler is anonymous');
+
 assert.sameValue(
   typeof secondArg,
   'function',
   'Invokes `then` method with a function as the second argument'
 );
-assert.notSameValue(secondArg, originalFinallyHandler, 'Invokes `then` method with a different fulfillment handler');
+assert.notSameValue(secondArg, originalFinallyHandler, 'Invokes `then` method with a different rejection handler');
+assert.sameValue(secondArg.length, 1, 'rejection handler has a length of 1');
+assert.sameValue(secondArg.name, anonName, 'rejection handler is anonymous');
 
 assert.sameValue(result, returnValue, 'Returns the result of the invocation of `then`');