[test] Fix {assertNotSame} in mjsunit test harness.

This assertion was borked, as it accepted obviously "same" values like
the same object. This fixes the predicate by switching both assertSame
and assertNotSame to use {Object.is} underneath. It also adds a new
respective regression test (gotta test the tester).

R=ahaas@chromium.org
TEST=message/mjsunit/fail/assert_not_same

Change-Id: I6ba20c4b8b96a736ab924715b1cad78f2f43a120
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1687541Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62512}

test/message/mjsunit/fail/assert_not_same.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load("test/mjsunit/mjsunit.js");
+
+assertNotSame(1, 1);

test/message/mjsunit/fail/assert_not_same.out

+test/mjsunit/mjsunit.js:{NUMBER}: Failure: expected <not same as 1> found <1>
+
+Stack: MjsUnitAssertionError
+    at assertNotSame *mjsunit.js {NUMBER}:{NUMBER}
+    at               *%(basename)s 7:1
+    throw new MjsUnitAssertionError(message);
+    ^
+MjsUnitAssertionError
+    at assertNotSame *mjsunit.js {NUMBER}:{NUMBER}
+    at               *%(basename)s 7:1

test/mjsunit/mjsunit.js

@@ -390,25 +390,13 @@ var prettyPrinted;
   }
 
   assertSame = function assertSame(expected, found, name_opt) {
-    // TODO(mstarzinger): We should think about using Harmony's egal operator
-    // or the function equivalent Object.is() here.
-    if (found === expected) {
-      if (expected !== 0 || (1 / expected) === (1 / found)) return;
-    } else if ((expected !== expected) && (found !== found)) {
-      return;
-    }
+    if (Object.is(expected, found)) return;
     fail(prettyPrinted(expected), found, name_opt);
   };
 
   assertNotSame = function assertNotSame(expected, found, name_opt) {
-    // TODO(mstarzinger): We should think about using Harmony's egal operator
-    // or the function equivalent Object.is() here.
-    if (found !== expected) {
-      if (expected === 0 || (1 / expected) !== (1 / found)) return;
-    } else if (!((expected !== expected) && (found !== found))) {
-      return;
-    }
-    fail(prettyPrinted(expected), found, name_opt);
+    if (!Object.is(expected, found)) return;
+    fail("not same as " + prettyPrinted(expected), found, name_opt);
   }
 
   assertEquals = function assertEquals(expected, found, name_opt) {