Enable integration tests on Windows using snapshotter and differ

Signed-off-by: Darren Stahl <darst@microsoft.com>

sys/filesys_unix.go

@@ -0,0 +1,10 @@
+// +build !windows
+
+package sys
+
+import "os"
+
+// ForceRemoveAll on unix is just a wrapper for os.RemoveAll
+func ForceRemoveAll(path string) error {
+	return os.RemoveAll(path)
+}