Mock open_file in D2::Core::Request::Upload namespace in test

On some (most?) platforms, altering the coderef for D2::FileUtils::open_file
via glob assignment does not alter where that code has been imported into
other namespaces, such as D2::Core::Request::Upload.

Mock D2::Core::Request::Upload::open_file to always croak (rather than
returning 0 and expecting it to behave like a file handle), and undo
the mock once finished that portion of the test.