Move async_file_test_helper.* to content/public/test/

BUG=338338

Review URL: https://codereview.chromium.org/157053010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251069 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251069