[bigint] Add a test for Bigint.prototype.valueOf.

We can already construct wrapper objects using Object().

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: Ic4079654ef1fcae2be4b588cb12c2645e199f4f7
Reviewed-on: https://chromium-review.googlesource.com/738089
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48955}

test/mjsunit/harmony/bigint/basics.js

@@ -230,8 +230,7 @@ const six = BigInt(6);
   assertEquals(Object(zero).valueOf(), another_zero);
   assertThrows(() => { return BigInt.prototype.valueOf.call("string"); },
                TypeError);
-  // TODO(jkummerow): Add tests for (new BigInt(...)).valueOf() when we
-  // can construct BigInt wrappers.
+  assertEquals(-42n, Object(-42n).valueOf());
 }
 
 // ToBoolean