You're reaching down to internals of `Yarp::Cache::Memcache`, thus creating a dependency to its implementation.
Using `#send` here is the telltale smell.

Also, it's really not obvious that hex string is the hash of `/abc`. That's a smell too, since you're making an assuption on which hash will be calculated from which path.

I suggest stubbing out `Memcache#fetch`, thus making this a "more unit" / selfcontained test.