Add object coercible tests for trim(start/End)

test/built-ins/String/prototype/trimEnd/this-value-not-obj-coercible.js

+// Copyright (C) 2017 Valerie Young. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: pending
+description: The "this" value must be object-coercible
+info: |
+  1. Let O be ? RequireObjectCoercible(this value).
+---*/
+
+var trimEnd = String.prototype.trimEnd;
+
+assert.sameValue(typeof trimEnd, 'function');
+
+assert.throws(TypeError, function() {
+  trimEnd.call(undefined);
+}, 'undefined');
+
+assert.throws(TypeError, function() {
+  trimEnd.call(null);
+}, 'null');

test/built-ins/String/prototype/trimStart/this-value-not-obj-coercible.js

+// Copyright (C) 2017 Valerie Young. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: pending
+description: The "this" value must be object-coercible
+info: |
+  1. Let O be ? RequireObjectCoercible(this value).
+---*/
+
+var trimStart = String.prototype.trimStart;
+
+assert.sameValue(typeof trimStart, 'function');
+
+assert.throws(TypeError, function() {
+  trimStart.call(undefined);
+}, 'undefined');
+
+assert.throws(TypeError, function() {
+  trimStart.call(null);
+}, 'null');