Use "long" paths prefixed with \\?\ on Windows. refs #2235

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Fix comments in test_copy_using_filecap to reflect what the tests actually do.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
Fix #2048 (test_copy_using_filecap was doing much more work than necessary).

Signed-off-by: Daira Hopwood <daira@jacaranda.org>