Fix MachO test which is failing on a Windows bot.

This is breaking http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/31647/steps/test%20lld/logs/stdio.

The issue seems to be that it can't write to a file in /tmp, probably because that path doesn't
exist on Windows.  This was failing after I added EXPECT_FALSE(ec) in r264961 for the error
handling migration.

llvm-svn: 264962