testing/avro-c: new aport

Hello! Kcat doesnt support yet the avro datatype, and in my company we are compiling and distributing a modified binary manually. I have prepared 2 packages and modification to Kcat to make it proper, and allow others to use it.

Here I would like to propose the first dependency: avro-c. (I could create an MR with all 3 changes (avro-c, libserdes, kcat modifications) if you like.)