Add some handle read/write helpers to mojo/common/test/test_utils.h

This helps to refactor raw_channel_posix_unittest to run on multiple
platforms.

TEST=mojo_common_unittests,mojo_system_unittests
BUG=None
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/172263006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252714 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252714