Fuchsia: Use low-level prng call, add test, update test runner.

runtime/bin/builtin_impl_sources.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This file contains all C++ sources for the dart:_builtin library and
 # some of the C++ sources for the dart:io library.  The rest are in
 # io_impl_sources.gypi.
 {
   'sources': [
     'crypto.cc',
     'crypto.h',
     'crypto_android.cc',
     'crypto_fuchsia.cc',
     'crypto_linux.cc',
     'crypto_macos.cc',
+    'crypto_test.cc',
     'crypto_win.cc',
     'builtin_common.cc',
     'dartutils.cc',
     'dartutils.h',
     'directory.cc',
     'directory.h',
     'directory_android.cc',
     'directory_fuchsia.cc',
     'directory_linux.cc',
     'directory_macos.cc',
@@ skipping 41 matching lines @@
     'thread_win.h',
     'utils.h',
     'utils_android.cc',
     'utils_fuchsia.cc',
     'utils_linux.cc',
     'utils_macos.cc',
     'utils_win.cc',
     'utils_win.h',
   ],
 }