Add Promise#finally invoked on thenable test (#2312)

* Rename proxy context test

* Add thenable context test

test/built-ins/Promise/prototype/finally/this-value-thenable.js

@@ -0,0 +1,17 @@
+// Copyright (C) 2019 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-promise.prototype.finally
+description: >
+  Promise.prototype.finally invoked on thenable returns result of "then" call.
+features: [Promise.prototype.finally]
+---*/
+
+var thenResult = {};
+var Thenable = function() {};
+Thenable.prototype.then = function() { return thenResult; };
+
+assert.sameValue(
+  Promise.prototype.finally.call(new Thenable()),
+  thenResult
+);