Refactor minidump test utilities for MinidumpWritable, …

minidump/minidump.gyp

 # Copyright 2014 The Crashpad Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
 # You may obtain a copy of the License at
 #
 #     http://www.apache.org/licenses/LICENSE-2.0
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 77 matching lines @@
         'minidump_thread_writer_test.cc',
         'minidump_writable_test.cc',
         'test/minidump_context_test_util.cc',
         'test/minidump_context_test_util.h',
         'test/minidump_file_writer_test_util.cc',
         'test/minidump_file_writer_test_util.h',
         'test/minidump_memory_writer_test_util.cc',
         'test/minidump_memory_writer_test_util.h',
         'test/minidump_string_writer_test_util.cc',
         'test/minidump_string_writer_test_util.h',
+        'test/minidump_writable_test_util.cc',
+        'test/minidump_writable_test_util.h',
       ],
     },
   ],
 }