[test] Add test for Turbofan module namespace access

Turbofan support for property loads from module namespace objects
has been tested by the test/js-perf-tests/Modules/basic-namespace
benchmark, but so far not by the mjsunit suite. This CL adds such
a test.
This is a follow-up to 8d7379c0.

Change-Id: I3c4183d761693199e6bc8740b812279efcd791a0
Reviewed-on: https://chromium-review.googlesource.com/1142594Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54535}

test/mjsunit/harmony/modules-import-namespace.js

+// Copyright 2018 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.
+//
+// MODULE
+
+// Flags: --allow-natives-syntax
+
+import * as m from "modules-skip-3.js";
+
+function get() {
+  return m.stringlife;
+}
+
+assertEquals("42", get());
+assertEquals("42", get());
+assertEquals("42", get());
+%OptimizeFunctionOnNextCall(get);
+assertEquals("42", get());