Reverts to using tmpfile() for non-Windows platforms. On google3
the test directory does not have write permissions, and hence the
Y4mWriteTest fails. This patch fixes the issue.
On Windows, a temporary file is created in the temp directory
that has write permissions.

The tests pass on linux, mingw, and MS visual studio.

Change-Id: Ibada1d80e25d8b8e5b6a9d3d597533674bd9024c