Fix FilePattern grouping Windows

This makes sure that file patterns are detected consistently, regardless
of how the file separator is escaped.

To test, run ```ant "-Dtestng.directory=\path\to\test_images_good\eps" test-automated``` on Windows without this PR, and verify that the ```SizeZ``` test fails.  With this PR, the same test should pass.

All builds are expected to remain green.  See also https://trello.com/c/0Rdx4AMl/113-nfs-on-windows