Add Object.prototype.toString tests for BigInt.

Reported by André Bargull in
<https://bugzilla.mozilla.org/show_bug.cgi?id=1464758>.

test/built-ins/Object/prototype/toString/symbol-tag-non-str-bigint.js

+// Copyright (C) 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-object.prototype.tostring
+description: Non-string values of `@@toStringTag` property are ignored
+info: |
+    14. Else, let builtinTag be "Object".
+    15. Let tag be ? Get(O, @@toStringTag).
+    16. If Type(tag) is not String, let tag be builtinTag.
+    17. Return the string-concatenation of "[object ", tag, and "]".
+features: [BigInt, Symbol.toStringTag]
+---*/
+
+let custom1 = BigInt(0);
+let custom2 = Object(BigInt(0));
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: undefined});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: null});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: Symbol.toStringTag});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: 86});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: new String('test262')});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: {}});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: _ => 'str'});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object Object]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object Object]');

test/built-ins/Object/prototype/toString/symbol-tag-override-bigint.js

+// Copyright (C) 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-object.prototype.tostring
+description: String values of `@@toStringTag` property override built-in tags
+info: |
+    15. Let tag be ? Get(O, @@toStringTag).
+    16. If Type(tag) is not String, let tag be builtinTag.
+    17. Return the string-concatenation of "[object ", tag, and "]".
+features: [BigInt, Symbol.toStringTag]
+---*/
+
+let custom1 = BigInt(0);
+let custom2 = Object(BigInt(0));
+
+Object.defineProperty(BigInt.prototype, Symbol.toStringTag, {value: 'test262'});
+assert.sameValue(Object.prototype.toString.call(custom1), '[object test262]');
+assert.sameValue(Object.prototype.toString.call(custom2), '[object test262]');