Change file_path_test.py to use fs.
Change file_path_test.py to use fs.

This fixes the problem on en_US systems where the default encoding is ascii.

R=mmeade@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1406293002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py