tests: fail if gf_methods is not found
tests: fail if gf_methods is not found

If the gf_methods was not found, the test would silently succeed doing
nothing. Check for existence and fail if it is not in the path.

Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>

cedricde/jerasure,windkit/jerasure,tsuraan/Jerasure,cedricde/jerasure,leo-project/jerasure,tsuraan/Jerasure,ceph/jerasure