[cleanup] Add message test for assertTrue in mjsunit.js

This is a first testing CL to add tests for mjsunit assertions to the
message tests.

R=clemensh@chromium.org

Bug: v8:7310
Change-Id: Ibbeafebb116f9608c70687f141986ab4534c6b2a
Reviewed-on: https://chromium-review.googlesource.com/803555Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51441}

test/message/mjsunit/fail/assert_true.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.
+
+load("test/mjsunit/mjsunit.js");
+
+assertTrue(false);

test/message/mjsunit/fail/assert_true.out

+test/mjsunit/mjsunit.js:{NUMBER}: Failure: expected <true> found <false>
+
+Stack: MjsUnitAssertionError
+    at assertTrue *mjsunit.js {NUMBER}:{NUMBER}
+    at            *%(basename)s 7:1
+    throw new MjsUnitAssertionError(message);
+    ^
+MjsUnitAssertionError
+    at assertTrue *mjsunit.js {NUMBER}:{NUMBER}
+    at            *%(basename)s 7:1