CDK-750. Fix AvroEntityMapperTest for JDK8.

I think this should also be a `Map<String, byte[]>` like the other. That way, we can pass in the bytes instead of prepending `(byte) 14` to the data. I think that's the string length encoded as a zig-zag int by Avro.

Nit: assertion methods usually take the expected value first and call the variable "expected".