Solution: move all corpora to the zeromq/libzmq-fuzz-corpora repository
for easier handling, and to avoid inflating the size of zeromq/libzmq.
Clone it for the CI on the fly.
Solution: port the 2 new tests from oss-fuzz, and wire them up to
be ran manually with a static input in normal builds.
Add a specific configure option to use the external fuzzing engine
from oss-fuzz.