test/py: Update test_fs to decode check_output calls

test/py: Update test_fs to decode check_output calls

The check_output function from the subprocess Python module by default
returns data as encoded bytes and leaves decoding to the application.
Given our uses of the call, it makes the most sense to immediately
decode the results.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>