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 <loic@dachary.org>
tests: add minimal encoder/decoder test

Add a test to run with make check to run encoder and decoder to make
sure they work at least in one simple case. It is also useful as a
documentation about how to use them.

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